Gemmology 1 PDF

What is Gemmology? September 12, 2024 7:33 PM - The study of the technical aspects of gemstones and gem materials - Identification of gem materials - Includes facetted, rough, gems set in jewellery - Country of origin - Synthetics and imitations...

What is Gemmology? September 12, 2024 7:33 PM - The study of the technical aspects of gemstones and gem materials - Identification of gem materials - Includes facetted, rough, gems set in jewellery - Country of origin - Synthetics and imitations - Treatments Why should we study it? - Identify gems - Apply safe handling in retail and repair situations - Selling - Identify more valuable varieties compared with imitations - Recognize natural materials from synthetics - Detect treatments Pre-Gemmology - 3BCE first observations about gems and minerals ○ Plato ○ Aristotle ○ Theophrasus - Pliny (died 79 ACE) - Islamic world, lbn Sina, 10-11ACE - 1500's microscope invented - Light and physics ○ Boyle ○ Brewster ○ Fraunhofer ○ Dieulfait - Synthetic materials 1889 - GEM-A 1908 Transition from art to science - Historically gems were prized for their colour, metaphysical properties, or ancient customs and beliefs - Colour was the primary method of identification ○ How to identify rubies ▪ Psychology of botany by Cahrubel 1906 ▪ The black prince's ruby (170 ct) featured on the Crown Jewels of England, was believed to be a ruby but was revealed to be spinel Tools for Gem Identification - Eyes - Loupe - Tweezers - Microscope - Refractometer - Polariscope Nomenclature of Gemstones - What is a Gemstone ○ Any material used for adornment or decoration Has value WEEK 1 Page 1  ○ Has value ○ Value is determined by (BARD) ▪ Beauty ▪ Acceptability ▪ Rarity ▪ Durability - Nature Gem Materials ○ Made by earth processes ○ No intervention by man ○ Includes minerals and organic materials ○ Genuine = a naturally occurring gem material which has not been altered other than by faceting and polishing - Crystalline Materials ○ Gem materials are made from atoms ○ In most gem materials these atoms are in orderly and regularly repeating arrangements ○ Different atomic arrangements are referred to as crystalline structures Mineral Gem Materials Crystalline Materials ○ A substance which has been formed within the earth by inorganic processes ○ Homogenous, uniform ○ Definite atomic structure ○ Set chemical formula ○ Physical and optical properties are consistent throughout - Inclusions ○ In natural materials that usually occur during the cooling and formation stage ○ Crystals, liquid, or gas filled cavities enclosed in a host material ○ Distinctive for natural materials and synthetics ○ Best viewed under magnification - Polycrystalline Materials ○ Material composed of an aggregate of randomly oriented small crystals and fibers - Crypto-crystalline or Microcrystalline Materials ○ Material composed of an aggregate of sub microscopic materials WEEK 1 Page 2  - Amorphous material ○ No definitive internal structure ○ Properties are the same in all directions - Organic/Biogenic Gem Materials ○ Produced by living organisms ○ With the exception of pearl and coral, these materials are non-crystalline - Synthetic Materials ○ Created by lab processes ○ Have the same chemical composition as their natural counterpart ○ Physical properties are very similar to the natural counterpart - Imitation Materials ○ Resemble the natural material ○ Are not chemically or physically the same as the natural material ○ Often glass or plastic ○ Natural materials can imitate other natural materials ex. Topaz for aquamarine - Rough Materials ○ Natural crystals, partial crystal, or irregular shape, unpolished - Rough crystal ○ Mineral ○ Shows crystal shape ○ No cutting or polishing Naming of Gemstones - Groups of minerals have similar features and characteristics - Species have their own individual chemical composition and characteristics - Varieties of species differ from each other only in colour or general appearance WEEK 1 Page 3 WEEK 1 Page 4 Gemstone Pipeline and Manufacturing September 18, 2024 5:20 PM The Gemstone Pipeline - Mining ○ Takes place all over the world ○ Countries have different regulations ○ Some gem materials are bi-product of other mineral mining ○ Small or large scale - Diamond Cutting - Coloured Stone Mining ○ Geologically quiet ○ Found in parent rock or in placers usually as unconsolidated gravels ○ Deposits are patchy and unpredictable ○ Remote, inhospitable terrain ○ Usually mined by individuals or small groups with simple tools - Unconsolidated Gravels ○ Abundant ○ Easy to mine ○ May have been worked for centuries ○ May be waterlogged, danger of collapse ○ Up to 20 meters below surface ○ Open pits or shafts (horizontal tunnels run off shaft) - Mining Tools ○ Picks, drills ○ Hard physical labour ○ Explosives to break up parent rock ○ Sieves and water for hand sorting ○ Larger excavation equipment is sometimes used - Modern Mining ○ More systematic and mechanized ○ Boreholes to determine yield ○ Topsoil removed, stored and replaced ○ Sorting by rotating drums, pulsating jigs ○ Moving equipment WEEK 1 Page 5 Important Coloured Stone Mining Localities - Sri Lanka (Ceylon) ○ Sapphire, spinel, chrysoberyl, tourmaline, zircon, garnet - Brazil ○ Tourmaline, aquamarine, chrysoberyl, topaz, citrine, amethyst - Myanmar (Burma) ○ Rubies, jade - Madagascar ○ Ruby, sapphire, topaz, garnet, zircon, chrysoberyl Diamond Mining - Gem quality and industrial quality are mined - Large scale - More mechanized than coloured stone mining - After ore is retrieved ○ Ore is crushed ○ Separated through heavy media separation (HMS) ○ Heavy media is pass through x-ray then over grease belt ○ Diamonds are sorted - Volcanic, kimberlite, or lamprolite ○ 100 tonnes of kimberlite ore = 25 carat diamond = 5 carats gem quality ○ Placer deposits, river and beach gravels ▪ Namibia 150 tonnes of sand and gravel = 5 carats nearly all gem quality Cutting and Polishing Gemstones - Lapidary ○ Cutting materials other than diamond ○ Describes the process and the person - Diamond Manufacture ○ Cut diamonds only ○ Diamond cutters and manufacturing - Factors in Choice of Cut ○ Yield ○ Transparency ○ Fire and brilliance ○ Inclusions ○ Optical effects ○ Colour ○ Cleavage and hardness directions ○ Durability ○ Desirability Yield - Retention of weight WEEK 1 Page 6 Transparency - Transparent = faceted - Opaque and translucent = cabochons, beads, carvings, inlays Fire and Brilliance - Specific proportions to display dispersion and brilliance - Total internal reflection in diamond Inclusions - Often heavily included materials are cut in a cabochon - An inclusion may also be displayed by the cut Optical Effects - Cabochons assist in enhancing reflections to create optical effects Colour - How colour is seen through table ○ Colour variations ○ Depth of colour ○ Pleochroism Cleavage and Directional Hardness - Crystalline structure and internal atomic structure dictate how a material is cut Durability - Style of cut offers protection - Stones with a hardness of less than 6 on Moh's hardness scale usually cut as cabs Desirability - Styles subject to design and fashion - Uniqueness - Famous cutter Slabbing and Trimming - Outlines are marked on rough material - Unwanted sections are sawn off using various cutting wheels Preform - Rough outline of final shape is created = preform - Use grinding wheels and laps 6-8 inches in diameter - Preform is then mounted on dop stick for final faceting and polishing Cabochons - Preform stone is mounted on the dopstick - Successive shaping and polishing using finer grits creates a lustrous surface - Laps include metal, plastic, felt - Polishing compounds include cerium oxide, aluminum oxide, diamond grit, carborundum, and rottenstone WEEK 1 Page 7  - Polishing compounds include cerium oxide, aluminum oxide, diamond grit, carborundum, and rottenstone Faceting Gems - Preform is cemented on a dop stick, metal, or wood with shellac - Overall shape and facet arrangement is created - Several grades of polishing wheels and grits are used to produce a lustrous finish Mounting the dop stick - Jamb-peg method - Tapered holes in wooden block at different heights - Dop is rotated and moved to various holes - Cutting is done by eye, no measurements - Experienced cutters are quick and efficient Mounting the Dop Stick - Mechanical faceting - Arm and angle can be rotated - Provides accurate cutting but is more time consuming to set up - Less flexible in cutting for weight retention General Faceting Process - Mount pre-form on dop - Cut pavilion facets and girdle ○ Coarse lap ○ Finer lap ○ Finest lap, polish pavilion - Transfer stone to dop stick mount on pavilion - Cut the crown facets ○ Coarse lap ○ Finer lap ○ Finest lap, polish crown facets - Cut the table ○ Coarse lap ○ Fine lap Finest lap, polish the table WEEK 1 Page 8  ○ Finest lap, polish the table Machine Cutting - Computer controlled simultaneous cutting - Preforms are mounted on dopsticks - 50 or more at a time - Produces calibrated sizes and shapes Drilling - Creates beads, holes for finding - Diamond drills or lasers can be used - Most commonly cutting is done by oscillation ○ High frequency ○ Metal tool vibrates ○ Abrasive grit and coolant used - Various hole shapes can be produced Tumble Polishing - Rotating drums with various cutting grits and water - Coarse to fine cutting over weeks - Irregular rough = baroques - Preforms and simple carvings ○ Calibrated cabochons ○ Beads Carving - Materials with uniform consistency and good durability can be carved into various forms - Free form faceting - Sculptures Cameos and Intaglios - Hand carving provides fine detail, requires great skill ○ Various cutting tools used ○ Cutting patterns visible - Computer cutting, mass production ○ Matt, "snow" finish in recesses ○ Usually banded materials Diamond Manufacture - Designing ○ Stone is inspected ▪ Carat weight ▪ Cleavage ▪ Colour ▪ Clarity ○ Marks the material ○ A viewing window may be created ○ Digital imagery may be utilized - 4 stages ○ Designing ○ Dividing ○ Shaping ○ Faceting and Polishing Designing (shape) - Shape of rough, determines the style of cut - Also in combination with inclusions, colour, and carat weight - 50% may be lost from rough to faceted Designing (maximizing brilliance and fire) Designing (colour) - Diamonds may be tinted, some skin deep and some throughout - Cut variation can emphasize or play down the colour Designing (inclusions) - Noticeable inclusions require the orientation of the cut stone to maximize the return yet minimize the visibility of the incl usion Designing (drawing the lines) - Designer marks diamond with Indian ink or permanent marker - Considerations to design ○ Grain of diamond/directional hardness ○ Cleavage planes Yield WEEK 1 Page 9  ○ Yield Dividing - Cleave ○ Not commonly done today - Sawing ○ Popular method - Laser cutting ○ Becoming more popular Dividing (sawing) - Diamond saw with phosphor bronze cutting disc coated with diamond powder ○ Particles randomly oriented for maximum hardness - May take several hour to cut a 1ct stone - Due to differential hardness, it can only be sawn in cubic and dodecahedral directions Dividing (cleaving) - Only in 4 directions, parallel to octahedral planes - Cemented to a rod - Small notch or kerf is cut - Steel bade tapped to cleave - Fast and efficient but yield is usually sacrificed Dividing (Laser Sawing) - Divides in any direction - Beam 0.001mm in diameter - Vapourized - Computer controlled - 1ct in 20 minutes Shaping - Bruting - Shaping the girdle by abrading diamond against another - Divides stone mounted on lathe and rotated - May also be done with laser - Fancy shapes outlined on polishing lathe - Modern method, electronically controlled WEEK 1 Page 10  Modern method, electronically controlled Faceting and Polishing - 3 parts ○ Scaife - cast iron disc, approx. 30mm coated with diamond grit and oil ○ Dop - holds the diamond and allows it to be rotated and inclined ○ Tang - holds the dop, forms tripod - Polishing grain ○ Randomly oriented diamond crystals make process possible, differential hardness ○ Some directions easier to cut ○ Difficult to cut in cleavage plane, must incline to this plane - Grinding and polishing ○ Cross worker grinds and polishes first 18 facets ▪ Table ▪ Four corners and bezels on crown ▪ Four corners and four pavilion facets and culet (if any) ○ Brillianteer ▪ Adds 40 remaining facets □ 24 on crown □ 16 on pavilion Automatic Polishing Machines - 1970's - 0.2ct to 10ct - Large and valuable, hand cutting preferred WEEK 1 Page 11 Examining and Documenting Gemstones September 12, 2024 7:35 PM Measure the Stone - For stones less than 25mm use the leverage gauge - Large stones may be estimated Describe the Colour - Main body colour - Modifying body colour - Tone - Hue - Saturation - Examine with multiple light sources Describe Cut - Outer shape - Faceting or cut style Make and Symmetry - Facet and surface condition - Symmetry and arrangement of facets - Girdle thickness - Crown and pavilion proportions - Quality of the polish Luster - Assess the quality and quantity of the reflected light - Remember surface condition affects the amount of light - Types of Lustre ○ Metallic (metal-like) ○ Adamantine (diamond-like) ○ Sub-adamantine (almost diamond like but not as sharp) ○ Bright vitreous (glossy) ○ Vitreous (glassy or glass-like) ○ Sub-vitreous (almost glassy) ○ Greasy (grease-like) ○ Resinous (resin-like) ○ Pearly (pearl-like) ○ Silky (silk-like) Transparency - Hold stone up to light - Don’t comment on reflections - If a material is heavily included it may be "transparent to translucent due to inclusions Eye Visible Inclusions - Colour and shape if visible e.g. Rutile needles in rutilated quartz - "Heavily included" - "Minor inclusions visible" WEEK 1 Page 12 Phenomena or Colour effects - Describe the effect, do not name it e.g. blue sheen Diagrams - Quick diagrams that show the stone outline and locations of important observations are required WEEK 1 Page 13 Describing Gemstone Colour September 12, 2024 9:06 PM Hue, Modifications of Pure Colour - Not all hues are pure - Combinations are possible - Colours can be modified by colours next to them or shades like brown or grey Hue, Pure Colour - First impression of any colour - Red, blue, green, yellow etc. WEEK 1 Page 14 Gemstone Shapes and Cuts September 12, 2024 9:11 PM WEEK 1 Page 15 Cabochon (cab) - Have a curved, convex top - Bottom may be flat (single cabochon) or convex (double cabochon) - May be transparent or opaque materials - Describe outside shape and profile or side view e.g. high round cabochon or oval double-bombe Buff Top - Cabochon cut with a facetted pavilion Tumbled Gem Materials - Polished and rounded by a mechanical rotary tumbler - Usually irregularly shaped - Transparent or opaque materials WEEK 1 Page 16 Gem Material Slices Tablets or Slabs - Flat with parallel surfaces - Describe colour, approx. shape and patterns if any Cameo - A low relief carving - Often made with material with alternating colour layers Intaglio - Carved into a gem material - Recessed Inlay - Gemstone cut to fit in a recess ○ Other material may be wood, metal, another gemstone ○ Excess material is polished off Intarsias and Mosaics - Small pieces fitted together - Top is fit flush to surface of base material then polished Screen clipping taken: 2024-09-12 9:24 PM Beads - Gem material with a drill hole - May be facetted, shaped, or rough - Describe colour and shape Spheres - Round shape without drill hole Carvings and Ornamental Items - Fashioned or carved three dimensional items To describe a gem shape: - Shape e.g. round - Type of fashioning e.g. faceted, cabochon, carving WEEK 1 Page 17 - Type of fashioning e.g. faceted, cabochon, carving - Details of cut e.g. round brilliant cut, round mixed cut WEEK 1 Page 18  Magnification September 12, 2024 9:28 PM - Useful for: ○ Identification ○ Detection of treatments - Eye Loupes ○ 10x or higher ○ Angular magnification ○ Focal length is about 25mm/1inch (10x) ○ Transportable ○ Inexpensive ○ Set or unset materials ○ Rough material - First test that a gemmologist completes - Doublets or triplets - Must be colour corrected (achromatic) - Must prevent distortion (aplanatic) - Test on graph paper - Use of the eye loupe ○ Hold stone close to the eye ○ Good lighting required ○ Put elbows on table ○ Keep hands together ○ Hold stone just below the rim of a desk lamp ○ Pen lights work well and are easily adjusted Observations under 10x magnification - Surface damage ○ Chipping scratches, surface flaws ○ Quality of the cut - symmetry ○ Quality of the finish (polishing lines) ○ Sharpness of facet junctions ○ Fractures ○ Incipient cleavage ○ External blemishes - Inclusions ○ Crystals, silk, veils, bubbles ○ Colour distribution and growth zones ○ Curved growth lines in synthetic materials ○ Strong double refraction ○ Composite stones (change in lustre, junction planes, bubbles, dust, or cement lines - The Microscope ○ Stereomicroscope ○ 10x to 75x - Stereo zoom binocular gemmological microscope ○ Ability to enlarge items that are not visible under 10x magnification ○ Lenses are grouped to avoid image and colour distortion ○ Magnification is changed by turning the zoom ring or knob ○ The greater the magnification, the narrower the depth of field ○ Gemstones can be easily manipulated to view different details while being held in tweezers (reposition for maximum viewing) - Illumination ○ Bright field, dark field and top illumination ○ Dark field provides best viewing of inclusions ○ Top illumination works well on opaque materials ○ Fibre optics lights work well to introduce precise lighting ○ Difference in lustre are more easily distinguished with top lighting ○ Vary your focus position of the object, light diffusion and diaphragm - Inclusions are… ○ Characteristics which are entirely inside a stone or that extend into it from the surface ○ Types of inclusion ▪ Solid, liquid, or gas ▪ Zoning and colour distribution ▪ Twinning ▪ Internal fractures and cleavage ▪ Treatments (laser drilled diamonds) WEEK 1 Page 19 Weekly Readings or Videos September 13, 2024 6:29 AM Gemmology, Peter Read Chapter 1 Chapter 2, pages 13-21, Geology Chapter 4, Crystallography Chapter 19, Gemstone Cutting styles, pages 243-259 Pages 141-143, Loupe Pages 44-46, cleavage and parting with polishing From Chapter 1 Introduction - Evolution of the science of gemmology ○ The science of gemmology is concerned with the study of the technical aspects of gemstones and gem materials and the use of these aspects for identification purposes ○ For well over 2000 years, philosophers and scientists have been captivated by the beauty and the enigma of gems, and down the years have left records of their observations on these ornamental products of nature. ○ One of the first gemstone books in English was written by Thomas Nichols as long ago as 1652 ○ It was only in the last half of the nineteenth century that the science of gemmology began to emerge as a specialized offshoot of an already well-established branch of science, mineralogy Highlights of the last 170 years - the first attempt at gemstone synthesis in 1837, the French chemist Marc Gaudin managed to grow some small crystals of ruby by melting together potassium aluminium sulphate and potassium chromate - This was in the period when there was much interest in reproducing the growth of crystalline substances, and when the first experiments were being made to dissolve the constituents in a solvent ‘flux’ of lower melting point - Sir Arthur Church in 1866 to the learned English periodical, The Intellectual Observer, he describes his experiments with an early spectroscope and his discovery of absorption bands in the spectrum of Ceylon zircons and almandine garnets - 1932 that a comprehensive study of gemstone spectra for identification purposes was to be undertaken by Basil Anderson - Some five years after Church’s letter appeared in print, the South African diamond rush was in full spate, and 5000 diggers were reported to be working along the banks of the Vaal, Modder and Orange rivers - In 1873, the primitive mining town that had sprung up around the site of the De Beers farm was formally named Kimberley after the British Secretary for the Colonies, the Earl of Kimberley - In 1877, the French chemist Edmond Fremy manufactured the first synthetic rubies of commercial quality. These crystals were grown in a large porcelain crucible containing a lead oxide flux in which was dissolved alumina powder mixed with a trace of a chromic salt ○ However, the resulting crystals were small and expensive to produce and were therefore no threat to natural rubies. The following year was marked by the discovery and identification of a new gem variety, green demantoid garnet - 1885, a quantity of relatively large ‘Geneva’ rubies appeared on the market. Initially, the presence of bubbles within the rubies proved them to be synthetic. Later, however, it was thought that the stones had been produced by fusing together smaller fragments of natural ruby, and for this reason they were then called ‘reconstructed’ rubies. More recently, analysis of surviving specimens has shown that such a fusion of small stones could not possibly have produced transparent rubies, and that the Geneva ruby was probably created by the multi-step melting of a mound of ruby powder in a flame - In South Africa, Cecil Rhodes and Barney Barnato had finally agreed to amalgamate their holdings, and a controlling company, De Beers Consolidated Mines Ltd, was incorporated in 1888. The name was taken from the De Beers brothers’ farm which had become the site of the famous ‘Big Hole’ of Kimberley. In the same year the first successful synthesis of gem quality emerald crystals was achieved by the French chemists Hautefeuille and Perrey using a flux process - The pace of gemstone synthesis began to quicken towards the turn of the century. In 1891, the French scientist Vernueil, a former assistant to Fremy, was perfecting the furnace he had designed for the production of synthetic corundum. Over 100 years later, furnaces of this type were to be producing in excess of 1000 million carats of synthetic corundum per annum worldwide. The year 1902 saw the discovery and documentation of the pink kunzite variety of spodumene, and three years later the 3106 carat Cullinan diamond was prised out of a sidewall in the opencast workings of the Premier mine near Pretoria in South Africa. In the same year in England, Dr Herbert Smith produced his first refractometer WEEK 1 Page 20  near Pretoria in South Africa. In the same year in England, Dr Herbert Smith produced his first refractometer (Figure 1.1), providing gemmologists at last with an instrument specifically designed for measuring the refractive index of gemstones (this was followed in 1907 by a larger brass version). By 1910, the first synthetic rubies produced by the Verneuil method appeared on the market, although ironically, these could not be identified as synthetic by means of the refractometer. - One event that did more than any other to establish gemmology as a serious science was the formation of the Gemmological Association of Great Britain, which began its life in 1908 as the Education Committee of the National Association of Goldsmiths - The Association held its first membership examinations in 1913, just three years after synthetic Verneuil rubies appeared on the market. World War I broke out in 1914, and it was not possible to reinstate the Association’s examinations for another eight years - in 1925, when Basil Anderson, fresh from university with degrees in chemistry and mineralogy, was engaged by the Diamond, Pearl and Precious Stone Section of the London Chamber of Commerce to set up a pearl testing laboratory in Hatton Garden. The urgent need for such a laboratory arose from the rapid growth of the Japanese cultured pearl industry and the problems that jewellers were experiencing in distinguishing native pearls from the cultured product - The growing importance of the refractometer to gemstone identification is indicated by the introduction in 1925 of yet another version. This new model (Figure 1.1), designed by the famous jeweller gemmologist B.J. Tully, used a rotatable hemisphere of glass - the first synthetic spinels were produced by the Verneuil method, and an endoscope pearl tester (Figure 1.2) was installed in Basil Anderson’s new Hatton Garden Laboratory. This equipment, brought over from France, made it possible to test over 200 pearls an hour. By 1928, when C.J. Payne joined the laboratory, nearly 50 000 pearls were being examined each year. The laboratory moved to new premises in 1928, and an X-ray unit was installed so that undrilled pearls could be tested using diffraction techniques. So began gemmology’s practical service to the jewellery trade, first with the identification of pearls (well over 4 million were tested in the laboratory which was merged much later with the Gemmological Association) and then with the detection of the new Verneuil synthetic rubies and spinels - Another milestone event occurred in 1931. Robert Shipley, who had been awarded the British Gemmological Association’s Diploma in 1929 and who had then pioneered his own gemmological correspondence course in the USA, founded the Gemological Institute of America. In the mid-1930s he was joined by his eldest son, Robert Shipley Jr, who helped develop a series of gem testing equipment which included a gem microscope, a diamond colorimeter, a refractometer and a polariscope Other important research work by Anderson and Payne during the comparative leisure of the mid-1930s included a reassessment of the majority of gem constants. Several stable and relatively safe heavy liquids for the determination of specific gravity were also established. In 1933, Basil Anderson took over the Chelsea Polytechnic classes in gemmology. One of the students in his first Diploma class was Robert Webster. During this period, an emerald filter (called the ‘Chelsea’ filter) was developed jointly by the London laboratory and the gemmology students (Figure 1.3) WEEK 1 Page 21  students (Figure 1.3) - Early in 1935, news appeared in the London press of a synthetic gemstone having ‘all the qualities of diamond’ and capable of ‘deceiving 99 per cent of the experts’. Today the story has an all-too-familiar ring, but back in the 1930s, this new product caused quite a stir. The simulant behind the scare story was colourless synthetic spinel, which was thought to have been manufactured in Germany - The same news caused similar consternation when it appeared in the North American press. There was further dismay in the trade on the announcement of the successful synthesis of diamond in gem qualities and sizes by a Mr Jourado, a self-styled gem expert. The Jourado stone was identified as a spinel by Anderson, and reports by him of the stone’s characteristics appeared in several of the leading gemmological journals and jewellery magazines - in 1940 the American chemist Carroll Chatham also succeeded in growing gem-quality crystals. Although the method of manufacture was kept secret, the Chatham emeralds were close enough in character to the German Igmarald to indicate that they were grown by a flux process. (In a move to avoid the use of the word ‘synthetic’, Chatham finally obtained permission in 1963 from the US Federal Trade Commission to market his product as ‘Chatham created emerald’) - Synthetic rutile, the first of a series of man-made diamond simulants, appeared in 1948 and was marketed under the trade names ‘Rainbow Gem’ and ‘Titania’. In 1951, a new rare gem species was confirmed and named ‘Taaffeite’ after its discoverer, Count Taafe. X-ray and chemical analysis were used to verify its principal constituents as beryllium, magnesium and aluminium (except for double refraction, taaffeite closely resembles spinel). Although still a rare species, a few taaffeites have since been found in Sri Lanka - In the 1950s, strontium titanate was introduced as yet another diamond simulant under the trade names ‘Fabulite’ and ‘Diagem’. Unlike the earlier simulant, synthetic rutile, there appeared to be no counterpart for this material in nature. During this period, progress was also being made in the field of diamond synthesis - The first experimental reflectance instrument for the identification of gemstones was developed in 1959 by L.C. Trumper, who was awarded a Research Diploma by the Gemmological Association of Great Britain for his thesis on the measurement of refractive index by reflection. His instrument took the form of an optical comparator in which the intensity of reflection from a gem’s surface was visually matched against a calibrated and manually adjustable source of illumination - In 1967, deposits of gem-quality transparent blue zoisite were discovered in Tanzania and given the name ‘Tanzanite’. Two years later, another two diamond simulants having no counterpart in nature were introduced. The first of these was yttrium aluminium garnet (YAG), which was marketed as ‘Diamonaire’ and ‘Diamonique’, and the second was lithium niobate, marketed as ‘Linobate’ - In 1970, synthetic gem-quality carat-size diamonds were grown under laboratory conditions by General Electric of America, but the resulting tabular crystals were not economically viable as a commercial product. In 1971, Russian research workers announced that they too had synthesized gem-quality diamond crystals but they also decided that their product was too costly to market. Identification challenges also appeared in the form of new diamond simulants. Gadolinium gallium garnet (GGG) was marketed in 1973, and cubic zirconium oxide (CZ), which virtually superseded all previous diamond simulants, appeared in 1976 - During this period, the need for help with the identification and appraisal of gems at the jeweller level resulted in a proliferation of new gem test equipment. Perhaps the most frequently used of these is the reflectance meter and the thermal conductance tester which have been developed mainly for the detection of diamond and its many simulants - The first commercial reflectance meters were marketed in 1975 (Figure 1.5), and the first commercial thermal tester appeared in 1978 (Figure 1.6). Because of the useful complementary features of these two types of test instrument, dual versions were introduced in 1984 WEEK 1 Page 22  instrument, dual versions were introduced in 1984 - De Beers announced the recovery (in 1986) of a 599 carat diamond from the Premier mine (Figure 1.7). The same year saw the introduction of the world’s first commercial computer program, Gemdata, for gemstone identification (developed by the author) - In 1990 The Gemmological Association of Great Britain ended its long partnership with the National Association of Goldsmiths and moved into separate premises in the Hatton Garden area of London. During the same year, the Gemmological Association merged with the London Gem Testing Laboratory to become the Gemmological Asso¬ ciation and Gem Testing Laboratory of Great Britain (GAGTL). In 2002, the GAGTL changed its logo to Gem -A - During the 1980s, 1990s and into the present century, there has also been an increase in the introduction of new gem enhancement techniques, from the heat treat¬ ments of corundum, the irradiation of yellow sapphire and blue topaz, the glass filling of fractures in diamond, ruby and sapphire, to the laser drilling of diamond and, in 1999, the colour improvement of brown Type II natural diamonds by HPHT anneal¬ ing techniques. High- temperature surface-diffusion and deep-diffusion of pale colour corundums is achieved by the addition of transition element oxides, and more recently the lattice diffusion of beryllium additives has produced orange and yellow sapphires but has also created new identification challenges The essential qualities of a gem material - Unlike a gem’s more tangible properties, its beauty cannot easily be quantified as it depends in the main on subjective factors to do with its appearance. If the stone is a trans¬ parent coloured gem, the depth of colour and degree of transparency will be the prime factors. However, in the case of a gem such as a diamond, beauty will be determined by its brilliancy, fire, optical purity and, in general, the absence of any body colour. With precious opal, the quality of its iridescent play of colour will be the deciding factor - Rarity is another quality which must be present in some degree in all gemstones worthy of the name (coloured glass, while it may be quite beautiful in terms of hue and transparency, is by no means rare). Unlike beauty, however, the rarity of a gem can be affected by factors such as supply and demand, and by both fashion and the scarcity of the source material. Amber and pearls have become popular again and therefore more expensive, while amethyst was once a rare and expensive gemstone until the dis¬ covery of the extensive Brazilian sources in the eighteenth century - he third essential quality which must be present in a gem before it can be con¬ sidered suitable for use in jewellery is its durability. This is a more practical quality than either beauty or rarity, but without it a gemstone would not be WEEK 1 Page 23  is its durability. This is a more practical quality than either beauty or rarity, but without it a gemstone would not be able to survive either the day-to-day wear and tear experienced by a piece of jewellery, or the chem¬ ical attack from pollutants in the atmosphere, and would soon lose its surface polish - Durability, which includes the property of hardness and toughness, is therefore a most important quality in a gemstone from the wearer’s point of view Organic and inorganic gems - Jewellery has, from the earliest times, included gem materials which have an organic as well as a mineral content, and because of this the science of gemmology today covers not only mineralogy, geology, optics and chemistry, but also overlaps into the fields of zoology, biology and botany - among the gem materials used in jewellery, the largest group is that of the mineral kingdom. The first part of this book therefore deals prin¬ cipally with the characteristics of gemstones having a mineral origin Chapter 2 (pages 13-21) The formation of rocks in the Earth’s crust - Sedimentary rocks ○ Returning to the Earth’s structure, the crust itself consists of three distinct layers (Figure 2.2). The upper and thinnest layer is composed mainly of the fine deposits of sand, grit and clay eroded by the action of rain, wind and flowing water from the ancient pre-existing rocks in the middle layer of the crust, and compressed to form layers of sandstone or limestone. Because of the mechanism of their formation, these top layer rocks are called sedimentary - Igneous rocks ○ The middle layer of the crust is formed by the solidification of molten magma. It is composed of rocks which are described as igneous (meaning ‘fiery’), and these are mainly granite. The lower layer also contains igneous rocks. These are darker and denser, and consist mainly of basalts, and other basic and ultra basic rocks low in silica content and rich in iron and magnesium. The igneous rocks which solidified in the middle and lower layers are called intrusive, plutonic or abyssal rocks, while those which erupted through the upper layer and were formed by the rapid cooling of magma at the surface of the crust are called extrusive or volcanic (e.g. lava) - Metamorphic rocks ○ Some igneous and sedimentary rocks in the various layers of the crust have undergone changes as the result of heat and pressure, and these are called metamorphic rocks. Marble is a metamorphic rock which has been produced in this way from limestone. Other examples of a metamorphic reaction is where molten magmas were forced into cooler rocks, and where the temperatures and pressures caused by the large-scale shearing and crushing of rocks produced similar changes - Mineral groups, species and varieties ○ In mineralogy, there are over 2000 different minerals. In order to subdivide these into manageable numbers they are split up initially into a series of groups, each of which contains those minerals which have similar features or characteristics. In gemmology, with its more limited number of gem minerals, there are only a few which have enough in common to qualify under this heading. These are the feldspar, garnet and tourmaline groups ○ Gem minerals are further subdivided into species, which have their own individual chemical composition and characteristics, and varieties of species, which differ from each other only in colour or general appearance. Starting with a group such as garnet, we can draw a family tree branching out through species such as andradite, grossular and spessartite, to their colour varieties - The origin of gems ○ The origin of a gem is its place of genesis in the Earth’s crust or mantle. Except for organic substances such as amber and jet, sedimentary rocks contain no primary gem material. However, if the pre-existing weathered rocks contained heavier minerals (i.e. gem minerals), these may have been washed out and swept away to form secondary or alluvial deposits. These gemstones often end up as water-worn pebbles, and can be seen in the gem gravels of Brazil, Myanmar (Burma) and Sri Lanka. Opal’s origin begins with low-temperature WEEK 1 Page 24  in the gem gravels of Brazil, Myanmar (Burma) and Sri Lanka. Opal’s origin begins with low-temperature silica-bearing water which solidifies as thin veins of silica gel material in the cracks and fissures of rocks ○ Most of our important gem minerals, such as feldspar and quartz, tourmaline, beryl, topaz and zircon, originated in intrusive or plutonic igneous rocks whose slower rate of cooling in the middle and lower parts of the crust made it possible for quite large crystals to form from the original molten residues (Figure 2.3). As the temperature of the magma began to drop, minerals separated out in a process known as fractional crystallization. The feldspars were the first to solidify, and having plenty of space they produced large well- shaped crystals ○ As the magma continued to cool, other minerals crystallized out - Of these, quartz was one of the last to solidify, and, as it had much less room than the others to grow, was not always able to produce such large and welldefined crystals. Many of the intrusive gem-bearing rocks formed as coarse-grained granites called pegmatites. Geodes also originated in igneous rocks in which quartz and other minerals have been precipitated as crystals in almost spherical cavities formed by chemical-rich molten or aqueous residues trapped in the magma (Figure 2.4) - he chemical reactions generated in metamorphic rocks when molten magma was forced into cooler rocks created the gemstone varieties of emerald, alexandrite, ruby and sapphire. Other gem minerals were formed as a result of the large-scale shear¬ ing and crushing of rocks. Examples of these are garnet, andalusite, serpentine, nephrite and jadeite WEEK 1 Page 25  - Diamonds (Figure 2.5) differ from the rest of the gem minerals in that they were formed somewhere in the region between the lower part of the Earth’s crust and the beginning of the mantle. This is the transitional zone at the base of the crust. It is known as the Mohorovicic Discontinuity or Moho (see Figures 2.1, 2.2), and is named after the Yugoslavian professor who discovered it. The current theory is that diamonds crystallized at least 70 miles (120 kilometres) beneath the Earth’s surface from carbon (in the form of carbon dioxide or methane) at very high temperatures and pressures. The diamond-bearing magma was then forced up to the Earth’s surface by explosive gas pressures in a volcanic-type eruption. The magma eventually cooled and solidified to form the present-day kimberlite pipes which make up the bulk of the world’s primary source of diamonds - The tops of these pipes are thought to have originally extended above the surface as cone-shaped hills or even as mountains. Over hundreds of millions of years these kimberlite hills were eroded away by the weathering action of wind and rain into low-lying hillocks or ‘kopjes’ (a South African word pronounced as ‘copy’). The diamonds contained in the eroded top section of the pipes were washed away to form alluvial deposits along river beds and, in the case of South West Africa, along the marine terraces of the Namibian coastal strip. - The manner in which gemstones were created in nature can therefore be related to the sedimentary, igneous or metamorphic processes of rock formation as well as the chemical content of molten and aqueous residues Gem Occurences - The occurrence of a gem is the geological environment in which it is found or mined (e.g. gem gravels, gem - bearing veins or a diamond pipe). Many gemstones are found at the site where they were originally formed, and this type of deposit is of particular interest to the mineralogist as it provides evidence of the method of gemstone formation. Alluvial gem deposits, on the other hand, are the result of gemstones which have been carried from their place of formation either by weathering agents such as wind or rain, or by rivers. Evidence of the distances travelled by gems found in alluvial deposits can be seen in their abraded surfaces (e.g. water-worn topaz pebbles and the rounded profiles of diamond crystals from the Namibian coastline). Sometimes, gems that were released from rocks by weathering have been deposited with little or no transportation or concentration from the site of the parent rocks. These residual deposits are described as eluvial The major gem localities - The locality of a gem deposit is the country or area in which it is found. Some areas of the Earth seem to have been more blessed than others by geological conditions favourable to the formation of gemstone deposits WEEK 1 Page 26 Mining techniques - The recovery of gems from the Earth’s crust varies from simple manually-intensive methods to highly mechanized operations. The mining of the alluvial gem deposits in Myanmar, Sri Lanka, Thailand and Brazil consists of sinking round or square section pits of up to 50 feet (15 metres) deep until a layer of gem gravel is reached (Figure 2.7, left). The gravel (known as ‘byon’ in Myanmar and ‘illam’ in Sri Lanka) is dug out and hauled or winched to the surface where the gem content, which is more dense than the sand and silt, is separated out by a rotary -motion hand-washing operation using a shallow woven basket (Figure 2.7, right). The sand and silt filters out through the fine mesh of the basket, and the lighter gravel content is spun out over the basket’s rim, leaving the heaver gem content to settle at the bottom. This concentrate (called ‘dullam’ in Sri Lanka) is then tipped out and picked over for its gem content WEEK 1 Page 27 WEEK 1 Page 28 - The methods employed in diamond mining range from the simple alluvial river bed operations just described (Figure 2.12), where the stones are separated from the gravels by means of a panning operation similar to that used when panning for gold, to highly mechanized shaft mining. Many of the diamond mine operations are concerned with pipe deposits, and for the first few years of their life these mines are worked as open-cast sites. As the top layers of the pipe are excavated using bulldozers and blasting to recover the diamond-bearing rock, so the country rock surrounding the top of the pipe has to be cut away to maintain a spiral roadway access to the working level Chapter 4 Crystallography - Crystalline and non-crystalline materials ○ All solid matter is composed of material which is either crystalline or non-crystalline*, or a mixture of these two states. Like many specialized terms, the word crystalline is derived from the Greek krystallos which means ice. The term gradually came to be used for any substance that had the clarity and transparency of ice, and is now also applied to crystals that are neither colourless nor transparent ○ In a non-crystalline substance, the atoms and molecules are positioned randomly throughout the material, and are not aligned in any special order or pattern. Because of this, a non-crystalline material can never develop any naturally occurring characteristic shape ○ Glass is a material of this type containing frameworks of disordered silicon tetrahedra. Because of the lack of overall crystal structure, glass cannot exhibit any of the crystalline characteristics such as external shape, or cleavage and pleochroism ○ Among gem minerals there are also a few which, although they are crystalline, do not have an identifiable external profile. Such minerals are called massive, a term which does not necessarily refer to their size or weight, but more to their lack of any characteristic shape. A common example of a massive crystalline gem mineral is the pink variety of quartz (i.e. rose quartz). Although mainly massive in external form, rose quartz does, however, very occasionally occur in the characteristic shape of a quartz crystal (other minerals which occur in the massive form are polycrystalline materials such as jadeite and nephrite, and microcrystalline materials such as agate and chrysoprase ○ Perhaps the most important feature of a crystalline material, and one that is missing from non-crystalline substances, is that its physical characteristics and properties vary with the orientation of the crystal. With a non-crystalline material, its properties are the same no matter what the direction of measurement or viewing, but in a crystalline substance they are related to the high degree of orderliness of its constituent WEEK 1 Page 29  viewing, but in a crystalline substance they are related to the high degree of orderliness of its constituent atoms and molecules ○ Diamond’s cleavage and hardness are two good examples of directional-dependant properties. Diamond can be cleaved or split only in the directions parallel to its octa¬ hedral crystal faces (Figure 4.1). Diamond’s hardness is also directional-dependant and because of this the stone is much easier to saw and polish in one direction than in others. This is an important factor that has to be taken into account by both the diamond sawyer and polisher who must avoid the cubic and octahedral planes of maximum hardness when fashioning this most durable of all the gem materials ○ As well as cleavage and hardness, optical properties such as colour can also vary with the direction of viewing in many crystalline materials. This can become a con¬ trolling factor in non-diamond coloured gems such as ruby, sapphire and tourmaline when the lapidary is deciding how the rough stone should be cut to bring out the best colour in the finished product - The atomic structure of a crystal ○ In 1669, Nicolas Steno, a Danish anatomist, made the fundamental crystallographic discovery that wherever a quartz rock crystal was found, its corresponding inter-facial angles were always the same regardless of the shape or size of the crystal. Subsequently, it was discovered that the angles between corresponding faces of other mineral crys¬ tals were also unvarying and specific to each individual mineral. As a result, Steno’s observations proved to have a universal application - Classification of crystals by symmetry ○ In crystallography, the concept of symmetry as applied to the crystal structure is of prime importance. As we will see shortly, crystals are classified into seven systems which range from the most symmetrical, the cubic or isometric system, to the least sym¬ metrical, the triclinic system ○ The crystallographic axes act as a frame of reference for identifying the positions of the crystal faces. They achieve this by means of their mutual angles and relative lengths. They are best thought of as imaginary lines which run through the centres or junctions of crystal faces to meet at a point within the ideal crystal called the ‘origin’. The three elements of symmetry can be defined as follows ○ Axis of symmetry ▪ An imaginary line positioned so that when the crystal is rotated around it, the characteristic profile of the crystal appears two, three, four or six times during each complete rotation as indicated in Figure WEEK 1 Page 30  the crystal appears two, three, four or six times during each complete rotation as indicated in Figure 4.4. (There are usually several possible axes of symmetry in a crystal; these are described as two-fold, three-fold, four-fold or six-fold axes. Alternatively, they may sometimes be labelled as digonal, trigonal, tetragonal or hexagonal axes of symmetry) - Plane of symmetry ○ This is an imaginary plane though a crystal which divides it into two mirror-image halves (see Figure 4.5). A cube has nine such planes - Centre of symmetry ○ A crystal is said to possess a centre of symmetry when identical faces and edges occur on exactly opposite sides of a central point - The seven crystal systems and their elements of symmetry ○ As mentioned earlier, crystals can be grouped into seven systems. These can be further subdivided into 32 classes. The subdivision is based on the different degrees of symmetry (as specified by the elements of symmetry) of the crystals within each system. The seven crystal systems themselves are classified in terms of the number of their crystal axes, their relative lengths and the angles between them ○ The cubic system (Group 1, Figure 4.6) ▪ Crystals in this system have the highest order of symmetry and are also called isometric. The cubic system has three crystal axes (a 1, a 2 , a 3 ), all of which are of equal length fa) = a 2 = a 3 ) and intersect each other at right angles (90°). ▪ Axes of symmetry: 13 (six two-fold, four threefold, three four-fold) ▪ Planes of symmetry: 9 Centre of symmetry: 1 ▪ Common forms: cube, eight-sided octahedron, twelve-sided dodecahedron Examples: diamond, garnet, spinel, fluorite WEEK 1 Page 31  spinel, fluorite ○ The tetragonal system (Group 2, Figure 4.7) ▪ This has three crystal axes. The two lateral ones are of equal length (a, = a 2 ) and at right angles (90°) to each other. The third (principal or c) axis is at right angles (90°) to the plane of the other two and is shorter or longer than them (c ≠ a,) ▪ Axes of symmetry: 5 (four two-fold, one fourfold) ▪ Planes of symmetry: 5 Centre of symmetry: 1 ▪ Common forms: four-sided prism with square cross-section Examples: zircon, scapolite ○ The hexagonal system (Group 2, Figure 4.8) ▪ Contains four crystal axes, the three lateral ones are of equal length (a1 = a 2 = a 3 ) and intersect each other at 60° in the same plane. The fourth (or principal) c axis is at right angles to the other three and is usually longer ▪ Axis of symmetry: 7 (six two-fold, one six-fold) ▪ Planes of symmetry: 7 ▪ Centre of symmetry: 1 ▪ Common forms: six-sided prism ▪ Examples: beryl, apatite The trigonal (or rhombohedral) system (Group 2, Figure 4.9) WEEK 1 Page 32 ○ The trigonal (or rhombohedral) system (Group 2, Figure 4.9) ▪ This system (sometimes treated as a subdivision of the hexagonal system) has four crystal axes which are arranged in the same manner as in the hexagonal system. The symmetry of the trigonal system is, however, lower than that of the hexagonal system ▪ Axes of symmetry: 4 (three two-fold, one three-fold) ▪ Planes of symmetry: 3 ▪ Centre of symmetry: 1 ▪ Common forms: three-sided prism, rhombohedron ▪ Examples: calcite, corundum, quartz, tourmaline ○ The orthorhombic system (Group 3, Figure 4.10) ▪ This system has three crystal axes, all at right angles (90°) to each other and all having different lengths (a ≠ b ≠ c). The principal or c axis is the longest, and of the remaining two lateral axes, the longer b is known as the macro axis and the shorter a is called the brachy axis. ▪ Axes of symmetry: (three two-fold) ▪ Planes of symmetry: 3 ▪ Centre of symmetry: 1 ▪ Common forms: rectangular prism (prism with cross-section of playing card ‘diamond’), bipyramid comprising two four-sided pyramids joined at the base ▪ Examples: topaz, peridot, chrysoberyl, andalusite, sinhalite, zoisite ○ The monoclinic system (Group 3, Figure 4.11) ▪ There are three crystal axes in this system, all of unequal lengths (a≠b≠ c). The b axis, known as the ortho axis, is at right angles to the plane of the other two which cut each other obliquely. The longest of these is the c axis, and the one inclined to it (at an angle other than 90°) is called the a or clino axis Axes of symmetry: (one two-fold) ▪ Planes of symmetry: 1 ▪ Centre of symmetry: 1 ▪ Common forms: prisms and pinacoids ▪ Examples: orthoclase feldspar (moonstone), diopside WEEK 1 Page 33  ○ The triclinic system (Group 3, Figure 4.12) ▪ This system has three crystal axes, all of unequal lengths (a ≠b ≠c) and all inclined to each other at angles other than 90°. The longer lateral axis is called the macro, and the shorter is called the brachy as in the orthorhombic system ▪ Axes of symmetry: none Planes of symmetry: none Centre of symmetry: 1 ▪ Common forms: prism (tilted sideways and backwards) with pinacoids Examples: plagioclase feldspar, microcline feldspar (amazonite), rhodonite, turquoise (usually in microcrystalline aggregates) Crystal forms - A crystal form is composed of a group of crystal faces which are similarly related to the crystal axes. A form made up entirely of identical interchangeable faces is called a closed form (e.g. a cube or an octahedron). A form which is only completed by the addition of other forms is called an open form. An open form cannot exist on its own and must be completed by the addition of other open forms which act as suitable terminations to complete the crystal shape (see Figure 4.13) - A tetragonal prism, for example, is an open form whose top and bottom can be terminated by a pinacoid or by four- sided pyramids, the latter producing the closed form of a zircon crystal - The pinacoid open form (which consists of a pair of crystal faces which are parallel to two crystal axes and are cut by the third) can occur in several positions - When it terminates a prism, as in the case of the emerald crystal in Figure 4.8, its faces are parallel to the crystal’s lateral axes and it is called a basal pinacoid. Another open form is the dome, which is often found as the terminating face on a topaz prism. The dome is defined (in a somewhat complicated way) as a form whose two faces intersect the vertical c axis of a crystal and one lateral axis, but are parallel to the other lateral axis (see Figure 4.13) Crystal habits and their use in identification - The shape in which a mineral usually crystallizes is referred to as its habit. From the sketches illustrating the seven crystal systems it can be seen that minerals belonging to the same system can often have very different habits, despite the similarity of their internal crystal structures WEEK 1 Page 34 - Enantiomorph: a crystal which has mirror image habits and optical characteristics, occurring in both right- and left- handed forms (e.g. quartz - see Figure 4.14) - Hemihedral: a crystal which has only half of the full number of faces of its symmetry class. - Hemimorphic : a crystal which has differing forms at the opposite ends of its axis of symmetry. - Isomorphic : minerals exhibit isomorphism when they have identical external forms but differ chemically (e.g. the garnet group). - Isomorphous replacement : The replacement of one element in a mineral by another. While the same form and crystal structure is retained, this may cause wide vari¬ ations in the mineral’s physical properties (e.g. the garnet group). - Lamellar. A crystalline structure composed of straight or curved layers often due to intermittent growth or twinning. - Polymorph : Minerals which differ in shape but have the same internal composition (e.g. diamond and graphite; andalusite, kyanite and sillimanite). - Pseudomorph : a mineral which has adopted an external form other than its normal habit by copying, for example, the shape of a pre-existing crystal or organic structure - Twinned crystals ○ A twinned crystal is one which consists of two or more individual crystals which have grown together in a crystallographic relationship to produce a symmetrical shape. Twinning, which is very common with quartz crystals, usually occurs in one of two forms: contact twins and interpenetrant twins ○ Contact twins (Figure 4.15) occur when the twin-halves of a crystal have grown with one half rotated through 180° to the other half. Spinel and diamond often occur in this form. With diamond a contact twin is called a made. In some minerals repeated twinning of this type (known as ‘polysynthetic’ twinning) produces a lamellar struc¬ ture consisting of thin plates of alternate orientation. This often results in a symmetrical habit uncharacteristic of the gem’s crystal system (e.g. the pseudo-hexagonal twin¬ ning of chrysoberyl called trilling ). It also produces planes of false cleavage known as ‘parting’ planes. Contact twins can usually be identified by the re-entrant angles between them (see chrysoberyl trilling in Figure 4.15) WEEK 1 Page 35 - Polycrystalline and microcrystalline minerals ○ Some minerals such as jadeite and nephrite are described as polycrystalline. They consist of aggregates of randomly orientated small crystals or crystalline fibres which can be seen under magnification (or sometimes by eye alone). In addition there are many minerals which are composed of an aggregate of very much smaller crystals or crystalline fibres. The size of the crystals in these minerals is so minute that they can¬ not be detected by eye or even with the aid of a standard optical microscope. Such materials are described as microcrystalline or cryptocrystalline (from the Greek word ‘kruptos’ meaning hidden). Gemstone examples include the chalcedony varieties and turquoise Because of their internal structure, polycrystalline and microcrystalline minerals are massive in their habit. WEEK 1 Page 36  ○ Because of their internal structure, polycrystalline and microcrystalline minerals are massive in their habit. Most polycrystalline or microcrystalline gems are also semi¬ transparent or opaque, and the random orientation of their crystals or crystalline fibres results in optical properties which are significantly different from normal crystalline minerals. A word of warning here: although all polycrystalline and microcrystalline materials are massive because of their internal structure, there are several massive gem minerals that are neither polycrystalline nor microcrystalline (e.g. rose quartz, rhodonite and rhodochrosite) - Metamict minerals ○ There are some minerals which have experienced natural alpha-particle irradiation (either from their surroundings in the Earth or from radioactive impurities or constituents). As a consequence, the crystalline structures of these minerals have become damaged to a point where they are virtually non-crystalline. Such minerals are described as metamict Chapter 19, Gemstone Cutting styles, pages 243-252 The fashioning of gemstones With the possible exception of a few gems, such as pearl* and occasionally amber, most of our organic and inorganic gem materials have their appearance enhanced by some form of shaping or polishing. With translucent or opaque stones this may be as basic as an abrasive tumbling operation to develop a smooth and lustrous surface while retaining the baroque profile of the original rough material. Alternatively, stones of this type may be cut in the rounded cabochon form (Figure 19.1) to bring out their surface colour or banding as with malachite and agate. Another use of the cabochon style is to emphasize sub-surface sheen features such as chatoyancy and asterism (i.e. in cat’s eye stones such as quartz and chrysoberyl, and star stones such as ruby and sapphire). If the stone is translucent but dark, the cabochon can be shallow cut to lighten the colour. Sometimes the back of a cabochon is hollow cut to produce the same effect Although tumble polishing a rough stone improves its appearance, and the cabochon cut is appropriate for opaque and ‘speciality’ stones, most transparent gem minerals are faceted to enhance features such as body colour , brilliance (the surface and total internal reflection of light), fire (dispersion) and scintillation (the sparkles of reflected light produced when the light source or the gemstone is moved). - The surface component of a gemstone’s brilliance (i.e. its lustre or reflectivity) is related to its refractive index by Fresnel’s equation (see Chapter 9). However, the pro¬ portion of brilliance due to the light reflected from within the stone depends on the style of cutting, which in turn is partly dictated by the critical angle of the gem material Critical angle - WEEK 1 Page 37 Critical angle and the jewellery owner - The importance of a gemstone’s critical angle has an aspect not always appreciated by the owner of gem-set jewellery. If the pavilion facets of a gemstone are allowed to become coated with grease and soap, the result will be a reduction in the stone’s overall brilliance. This is because the RI of grease and soap is greater than that of air, and this will increase the gem’s critical angle (see equation). The effect is particularly noticeable in the case of a brilliant cut diamond, 83% of whose brilliance comes from total internal reflection, and is sufficient justification for cleaning jewellery occasionally in a mild grease solvent Cutting styles - With a colourless transparent stone, one of the main aims is to achieve a polished gem which has the maximum possible brilliance of appearance. As mentioned earlier, this brilliance is dependant both on the surface reflectivity or lustre of the stone (which with some stones is evident even in the rough uncut state) and on the total internal reflection of those rays entering the gem’s crown facets. If the stone has an appreciable degree of dispersion, the cut must also exploit this to bring out the gem’s ‘fire’. - The dispersion of white light into its spectral colours which produces the effect of fire (Figure 19.4) also reduces the amount of undispersed white light being reflected back from the pavilion facets. For this reason, the design of the stone’s cut has to strike a balance between fire and brilliance. Incident rays which meet the table facet at right angles (i.e. normal) to its surface are not refracted as they pass from the air into the stone, and are therefore not dispersed as is the ray shown in Figure 19.4. Diamond cuts - One of the earliest cutting styles for diamond was the point cut. This simply consisted of polishing the octahedral diamond crystal while leaving the basic shape intact (as the octahedral faces of a diamond represent the hardest planes and cannot be worked, the stone must have been polished at a slight angle to these faces) WEEK 1 Page 38  planes and cannot be worked, the stone must have been polished at a slight angle to these faces) - The zircon cut ○ This style, based on the ideal cut for diamond, is designed to improve the brilliance of a zircon (whose RI is somewhat lower than that of diamond). Although having the same crown/girdle and girdle/pavilion angles as diamond’s brilliant cut, the zircon cut reduces the amount of light leakage from the rear of the stone by employing extra facets which are placed between the culet and the main pavilion facets (Figure 19.12) - The emerald cut ○ Designed specifically for emerald, this cut not only enhances the stone’s colour, but allows for its shock sensitivity by omitting the otherwise vulnerable comers (Figure 19.13). The cut is also known as the ‘step’ or’ ‘trap cut’, and is often used for diamonds and other gems. A variant, called the radiant cut (Figure 19.14), is used specifically for diamonds - The scissors or cross cut ○ This is basically a modification of the emerald cut, but has a rectangular plan profile and triangular facets around the table (Figure 19.15). It is used, like the emerald cut, to enhance the colour of a stone while maintaining a reasonable degree of brilliance. There is, however, some loss of light through the centre area of the pavilion - The mixed cut WEEK 1 Page 39 - The mixed cut ○ This is mainly used for coloured gemstones. It consists of a brilliant-cut crown (oval or round) and a step- or trap cut-pavilion (Figure 19.16). The style is frequently used by native cutters and is often made with an over- deep pavilion in order to retain maximum weight in the stone - Other styles ○ There are a great variety of cutting styles in addition to the main traditional ones mentioned in the preceding paragraphs. Many of these are variants of the basic cuts and are designed to accommodate the final shape of the rough stone. Some of the simpler styles have descriptive names such as the hexagonal, square, triangular and keystone cuts. One of the more elaborate variants used for larger coloured stones is the Portuguese cut, which can have up to 177 facets (Figure 19.16). Laser equipment is increasingly being used to produce new cutting styles - Gemstone polishing ○ In the very early days of gemstone fashioning, a polisher or lapidary would cut and polish both diamonds and other gemstones. However, over the years the gemstone world has grown into two distinct and separate industries ranging from mining to polishing and marketing. Diamonds are now almost exclusively polished by diamond cutting specialists, and all the other gemstones are cut and polished by lapidaries. Although very rarely a lapidary may cut a diamond, a diamond cutter is always most reluctant to risk his or her polishing equipment on anything other than diamond - Lapidary techniques ○ The basic method of polishing a non-diamond gemstone is a combination of sawing and rough grinding by the cutter to produce the required profile or preform (with the surface of the stone left in a matt state), followed by a finishing operation by the polisher to achieve the final surface or facet lustre ○ With a piece of rough having strong pleochroism, chatoyancy or asterism, it will first be necessary to orientate the gem material correctly. For cat’s-eye and star stones the fibres, needles or cavities producing the sheen effect must be positioned parallel to the base of the cabochon. As mentioned in Chapter 5, the lapidary must also be aware of any planes of cleavage in his or her stone in order to avoid accidental breakage during polishing. The lapidary must ensure that any polished facet is tilted by at least 5° from such a plane. If an attempt is made, for example, to polish a facet at right angles to the length of a topaz prism, the result will be a pitted surface where layers of topaz have cleaved away from the stone (Figure 19.17) - Next, the rough gemstone material is ‘slabbed’ (i.e. sawn) into suitable pieces using a rotary saw whose cutting edge is impregnated with diamond grit. These pieces are then marked with the profile of the required finished stone. If a cabochon is to be produced, the sawn ‘blank’ is passed to the cutter who develops the rounded base outline using a water-cooled vertical grinding wheel. Following this operation the stone is cemented to a dop stick for easier handling, and the dome of the cabochon formed on another finer grinding wheel. The final polish is achieved on a horizontal rotary lap using an appropriate polishing powder. For cabochon polishing, the lap may be of wood and contain grooves having the necessary radii WEEK 1 Page 40 - WEEK 1 Page 41 Crystallography September 13, 2024 6:29 AM Crystalline and Amorphous - Crystalline Structure ○ Atoms are arranged in an ordered and symmetrical three-dimensional pattern or lattice ○ Same structure in an orderly repeating way ○ Looks the same mirrored ○ Visible in the external shape of a mineral specimen ○ Physical characteristics and properties vary with the orientation of the crystal ▪ Due to chemical and trace elements, the properties vary from the direction you are looking at it - Silica Tetrahedral in Mineral Formation ○ Quick cooling ▪ Wont form uniform gems ○ Long chains may form and link with metal ions ○ Produce a partially orderly structure with molten liquids such as magma and molten glass ○ Units become sticks or viscous and do not flow ○ Sudden cooling occurs and the units cannot fully form into completely organized structures ○ The time a gem takes to form affects the quality of the gem - Amorphous Structure ○ Without form (random) ○ Randomly oriented atoms, molecules, or minute crystalline sectors ○ The material has no overall crystal structure/form and no crystalline properties ○ Never pleochroic - Vitreous Silica ○ Silica = sand ○ Sand being vaporized ○ Amorphous structure ○ Lacks a defined pattern ○ Ex. Obsidian, tektites, and opal are 2 naturally occurring amorphous stones ○ Opal (natural silica) ▪ Mineraloid (looks like a mineral but isn't) - Man-made Glass ○ Made in the lab ○ Silica is melted and cooled quickly ○ Molecules do not fully align, disordered atomic arrangement ○ Dates to Egyptian, Faience WEEK 2 Page 42 WEEK 2 Page 43 Crystal Systems Intro September 13, 2024 6:52 AM Crystalline Materials - Crystal growth ○ Natural formation in the earth ○ Crystallization from aqueous or molten solutions ○ Large well shaped crystals ▪ Slow cooling from molten materials ▪ Slow cooling from aqueous solutions under high pressure and temperature ○ Small crystals and/or microcrystalline masses ○ Geodes, druse, or vugs - Crystalline Materials ○ Atoms are arranged in an ordered and symmetrical three dimensional pattern or lattice ○ Visible in the external shape of a mineral specimen ○ Physical characteristics and properties vary with the orientation of the crystal - Crystallography ○ The study of the science behind crystals ○ The unit cell or the smallest atomic or molecular structure which still retains the characteristics of the mineral ○ Atoms and a crystal lattice reproduce and grow onto each other in the same arrangements - Properties that vary with direction ○ Crystalline materials only ▪ Hardness ▪ Cleavage ▪ Colour ▪ Optical properties ○ Diamond can be split/cleaved in directions parallel to its octahedral crystal faces ○ Hardness varies with direction so sawing takes place parallel to its horizontal axis - Crystal Symmetry ○ Depends on how symmetrical the crystal is ○ Describes the repetition of the structural arrangement of the atomic bonds Classification of crystals based on symmetry - Reference axes used to describe ideal shapes - An imaginary line of indefinite length that runs through the centre or junctions of crystal faces to meet at a point within the ideal crystal called the origin - Repeat directions of the crystal lattice and relative repeat distance - Relative angles of the axes - Fold ○ How many times does it appear identical Crystallographic Axes - Axial cross - Three or four intersecting reference axes - Axes meet at the origin WEEK 2 Page 44  - Axes meet at the origin - Represent the whole set of directions that define the crystal lattice directions and distances - The vertical axis is referred to the c-axis (in most cases, the longest line) Elements of Symmetry - Axis of symmetry, rotation ○ An imaginary line which indicates a direction through a crystal structure about which the structure can be rotated to appear two, three, four, or six times during one complete (180 degree) rotation Appears identical 6 times when held in one direction Number of axis ○ Sometimes referred to as rotation axes - Plane of symmetry, mirror images ○ An imaginary mirror plane dividing a crystal structure into mirror parts - Centre of symmetry ○ The centre of symmetry when identical faces and edges occur on exactly opposite sides of a central point The Seven Crystal Systems 1) Cubic - Isometric system - Highest order of symmetry - Three crystallographic axes of equal length which intersect at 90 degree angles 4x3 fold - Axes of symmetry = 13 ○ 6x2 fold ○ 4x3 fold 3x4 fold WEEK 2 Page 45  - Axes of symmetry = 13 ○ 6x2 fold ○ 4x3 fold ○ 3x4 fold - Planes of symmetry = 9 - Centre of symmetry = 1 - Common forms ○ Cube ○ Octahedron ○ Dodecahedron 2) Tetragonal (rectangle with square end) - Three crystallographic axes - Two (lateral) of equal length and at right angles to each other - The c-axis is at 90 degrees to the lateral axes and longer or shorter than the lateral axes a1 = a2 All at 90 degrees - Axes of symmetry = 5 ○ 4x2 fold, 1x4 fold - Planes of symmetry = 5 - Centre of symmetry = 1 - Common forms ○ Square sided prism with a square cross section ○ Octahedron ○ Dodecahedron - Ex. Scapolite, rutile, zircon 3) Hexagonal (6 sided prism) - 4 crystallographic axes - Three lateral ones are of equal length, intersecting at 60 degree angles in the same plane - C-axis at 90 degrees to the lateral axes and is usually longer - Axes of symmetry = 7 ○ 6x2 fold, 1x6 fold - Planes of symmetry = 7 - Centre of symmetry = 1 - Common forms ○ Six sided prism - Ex. Red beryl/Bixbite, apatite, synthetic moissanite 4) Trigonal (3 sided prism) - Also referred to as the rhombohedral system - Sometimes treated as a subdivision of hexagonal - Same number of crystallographic axes with fewer planes and axes of symmetry - 4 crystallographic axes where the 3 lateral ones are of equal length and intersect each other at 60 WEEK 2 Page 46  - 4 crystallographic axes where the 3 lateral ones are of equal length and intersect each other at 60 degrees in the same plane - The c-axis is usually longer 60 or 120 degrees, Laterals at 90 degrees - Axes of symmetry = 4 ○ 3x2 fold, 1x3 fold (difference between hexagonal) - Planes of symmetry = 3 - Centre of symmetry = 1 - Common forms ○ Three sided prism ○ Rhombohedron - Ex. Calcite, corundum, quartz, tourmaline 5) Orthorhombic (matchbox) - Three crystallographic axes, all at right angles to each other, all different lengths - c-axis is the longest - Longest lateral axis is the macro - Shortest lateral axis is the brachy All at 90 degrees - Axes of symmetry = 3 ○ 3x2 fold - Planes of symmetry =3 - Centre of symmetry = 1 - Common forms ○ Rectangular prism ○ Bi-pyramidal prism - Ex. Peridot, tanzanite, iolite, topaz 6) Monoclinic (slanted matchbox) - Three unequal axis, the longest is the c-axis, the inclined (other than 90 degrees), is the a or clino axis - The b-axis, ortho axis, is at right angles to the other two which cut each other obliquely b at 90 degrees a not 90 degrees to c - Axes of symmetry = 1 ○ 1x2 fold Planes of symmetry = 1 WEEK 2 Page 47  a not 90 degrees to c - Axes of symmetry = 1 ○ 1x2 fold - Planes of symmetry = 1 - Centre of symmetry = 1 - Common forms ○ Prism ○ Pinacoid - Ex. Orthoclase feldspar (moonstone), jadeite, nephrite, kunzite spodumene 7) Triclinic (matchbox slanted in two directions) - Three axes of unequal length - All inclined to each other at angles other than 90 degrees - The longer lateral axis is the macro - The shorter lateral axis is the brachy - Axes of symmetry = 0 - Planes of symmetry = 0 - Centre of symmetry = 1 - Common forms ○ Prism tilted sideways and backwards with pinacoids - Ex. Amazonite, microcline feldspar, diopside, turquoise Crystal Habits - The overall shape in which a crystal usually forms - Some crystal shapes are open forms which are closed with suitable terminations Twinning - Crystals may be twinned to form new habits - When gemstone is growing, the abrupt change to temperature changes direction of growth WEEK 2 Page 48  GEMS SR CUBIC (most symmetrical) Singly refractive GONALS TETRAGONAL HEXAGONAL Double Refractive TRIGONAL ICS OTHOROMBIC Double Refractive DR (pleochroism) MONOCLINIC TRICLINIC (least symmetrical) - Always check if gem is SR/DR - MNEOMIC = C, THT, OMT WEEK 2 Page 49 Crystal Structure (forms and habits) September 13, 2024 3:55 PM Crystal Form - A set of external crystal faces related to the symmetry of the crystal - Closed crystal forms ○ Composed of identical interchangeable crystal faces ① shape ○ Cubic family ○ e.g. cube or dodecahedron ○ Has to be symmetrical ② shape - Open forms ○ Must be completed by the addition of other forms ○ Cannot exist on its own ○ Usually, other additional forms are the terminations Open Form and Closed Form Basal Pinacoid (2 flat planes to close it) Pedion (capped on one side) Crystal Habits - The basic shape ex. Octahedron - The development of a crystal form or the relative development of two or more forms - The shape in which a crystal usually forms (rough gems only) - Overall shape of a crystal - Some crystal shapes are open forms which are closed with suitable terminations Types of Habits - Acicular ○ Slender needle like shape - Anhedral ○ Poorly formed External faces are not developed WEEK 2 Page 50  ○ External faces are not developed ○ Massive is a type of anhedral ○ A condition different from botryoidal - Banded ○ Broadly layered texture ○ Ex. Agate Rhodochrosite - Bipyramidal ○ Hexagonal bipyramid ○ Two pyramids joined at base ○ Pyramid with a hexagonal bottom - Botryoidal ○ Sometimes referred to as globular ○ Hemispherical masses (resembles a bunch of grapes) Hematite - Cruciform ○ Cross shaped habit ○ May be a result of crystalline structure or twinning WEEK 2 Page 51  Chiastolite family of andalusite - Cube ○ 6 sided figure with 90 degree corners - Columnar or Fibrous ○ Series of slender prisms ○ Blade like shapes - Dendritic ○ Branching or tree-like feature or inclusion Agate - Dodecahedral ○ 12 sided dodecahedron ○ Ex. Types of garnet - Drusy ○ Minute (tiny) crystals coating a surface ○ Ex. Pyrite WEEK 2 Page 52  Sphalerite - Equant ○ Roughly equal in all directions ○ Cubic system only ○ Ex. Diamond, spinel - Enantiomorphic ○ Mirror image habits and optical characteristics ○ Right handed and left handed forms ○ Only used for quartz a) Left handed quartz b) Right handed quartz - Euhedral ○ A well formed crystal shape - Filiform Capillary ○ Hair or thread-like ○ Fine formations ○ Different from acicular which is rod-like - Foliated Micaceous ○ Layered structure that parts into thin sheets ○ Micaceous as in mica WEEK 2 Page 53 - Granular ○ Grain-like ○ Aggregates of poorly formed crystals in matrix Anhedral Massive - Hemihedral ○ Has only half of the full number of faces in its symmetry classification Ideal Emerald - Hemimorphic ○ Differing forms at opposite ends of its axis of symmetry 2 Different closed shapes WEEK 2 Page 54 - Mamillary ○ Rounded intersecting corner ○ Botryoidal - Lamellar ○ Thin layers ○ Repetitive ○ Type of twinning ○ Ex. Ruby, cabradorite, moonstone - Massive ○ Without external crystal shape ○ No individual grains or crystal visible - Octahedral ○ Eight sided octahedron ○ Bipyramid, euhedral ○ Diamond, cubic WEEK 2 Page 55 - Prismatic ○ Main faces are parallel to c-axis beryl - Pseudo-Hexagonal ○ Resembles hexagonal due to cyclic twinning ○ Orthorhombic ○ Ex.. Aragonite (pearl) - Radiant divergent ○ Radiated outward from a central point Disk ○ Only seen in pyrite - Reticulated ○ Crystal form a net-like growth WEEK 2 Page 56 - Rhombohedron ○ Six sided figure, no 90 degree angles - Rosette ○ Platy, radiating rose-like aggregate - Sphenoid ○ Wedge shaped Sphene - Scalenohedron ○ Six sided bipyramid with unequal sides ○ Spindle shape, only used to describe sapphire WEEK 2 Page 57  Rough sapphire - Striations ○ Growth lines on the surface of a crystal ○ Parallel or perpendicular to c-axis - Tabular ○ Short and stubby crystal ○ Tablet like ○ Ex. Rubies (no height) External Crystal Features - Growth marks, created when the crystal formed ○ Colour zoning or striations ○ Etch marks ○ Surface striations Lidocotite Named after Richard T. Lidocote (father of gemmology) WEEK 2 Page 58 WEEK 2 Page 59  Common Crystal Forms and Habits September 18, 2024 12:42 PM APATITE (Crystal System: Hexagonal) Colours and Varieties: - Yellow - Blue - Green - Violet - Colourless Prismatic Habit Pyramid Secondary Face (smaller face) ○ Hexagonal Prism (larger face) BERYL (Crystal System: Hexagonal) Colours and Varieties: - Colourless = Goshenite - Green = Emerald - Pink = Morganite - Yellow = Heliodor - Blue = Aquamarine - Red = Red beryl - Grey-Blue = Maxixe Pinacoid Hexagonal Prismatic Habit Etch Pits ○ Hexagonal Rectangular Bi Pyramids Etch Pits Hexagonal Prisms CALCITE (Crystal System: Trigonal) Colours and varieties: Commonly white/colourless, but may come in variety of colours as well as banded Cleavage Cracks Rhombohedron (plane of atomic weakness split easily) Habit ○  Double line Terrace-like (double refraction) Face markings WEEK 2 Page 60  Face markings CHALCEDONY, POLYCRYSTALLINE, OR MICROCRYSTALLINE QUARTZ Colours and varieties: Varied Botryoidal Habit ○ CHRYSOBERYL (Crystal System: Orthorhombic) Colour and varieties: - Yellowish-Green - Green - Brown = Chrysoberyl - Red/Green with colour change = Alexandrite  Trilling Structure is due to Habit (common habit name) Lamellar and Cyclical Aka. Pseudo Hexagonal Twinning CORUNDUM Colours and varieties: - All colours other than red is sapphire - Red = Ruby ► RUBY Triangular Tabular Habit Growth Marks WEEK 2 Page 61  Rhombohedron Lamellar Twinning forms alternate ► SAPPHIRE (2 VERSIONS) Scalenohedron Growth Marks Habit Barrel Shape Spindle Shape Both Hexagonal Bi-Pyramids Horizontal Striations DANBURITE (Crystal System: Orthorhombic) Colour and varieties: - Colourless - Straw yellow - Brownish white - White Orthorhombic Dome Prismatic Habit Vertical Striations Cross Section Lacks Basal Cleavage DIAMOND (Crystal System: Cubic) Colours and varieties: - Colourless - Yellow - Brown - Green Trigons, pop up/recessed - Pink (only mined stones) - Blue and black - Purple or red (rare) Dodecahedral Habit (rare) Grain line WEEK 2 Page 62  Dodecahedral Habit (rare) Grain line Twin/Macle Clipped Corners Herringbone Markings ORTHOCLASE FELDSPAR Colour and varieties: - Transparent = Adularia - Yellow to colourless with iridescence = Moonstone - Yellow = Orthoclase Cleavage Crack MICROCLINE AND PLAGIOCLASE FELDSPAR Colour and varieties: Microcline ○ Green = Amazonite Plagioclase ○ Multi-colour = Labradorite ○ Yellow = Oligoclase ○ Bronze/Green Spangled = Aventurine Feldspar ○ White, Green, or Brown = Albite Moonstone SAME DRAWING AS ABOVE FLUORITE (Crystal System: Cubic)