Things to Memorise

### Causes of Color in Gemstones
- Color in gemstones can arise from various mechanisms related to their chemical composition and crystal structure.

### Metal-to-Metal Charge Transfer
- This involves the transfer of electrons between metal ions within the gemstone's structure, causing selective light absorption.

### Oxygen-to-Metal Charge Transfer
-  A charge transfer mechanism where electrons move from oxygen to a metal ion, leading to color.

### Charge Transfer Without Metal Ions
-  Color can also result from charge transfer processes that don't involve metal ions directly.

### Vacancy Color Center
- Occurs when an atom is missing from the crystal lattice, the resulting electron utilizes photons of light to jump to a higher energy level.
- The observed absorption is in the red part of the spectrum, leading to a green color perception.

### Hole Color Center
- Created when irradiation displaces an electron from its orbit, enabling other atoms to absorb light thus creating colour.

### Electron Color Centers
- An electron becomes trapped in a vacancy within the crystal structure, resulting in color.

### Vibronic Color Centers
- In diamonds, nitrogen trace elements surrounding a vacancy cause absorption due to both color center and molecular vibrations.
- This combined effect results in the perception of blue and yellow colors.

### Plastic Deformation
- Internal strain within the stone, often caused by stresses in the Earth’s mantle, can lead to color changes.
- The exact mechanism is not well understood.

### Band Gap Theory
- The electron energy band gap refers to the energy difference between valence band electrons and conduction band electrons.
- The width of the band gap is influenced by the gem's chemistry and crystal structure.

### Band Gap Larger than the VIS Energy Range
- When the band gap is larger than the energy range of visible light, no light is absorbed.
- The material appears transparent and colorless if pure.

### Band Gap Lower than the VIS Spectrum
- If the band gap is lower than the visible spectrum, all visible light is absorbed.
- The material appears opaque, with most light immediately re-emitted, creating a shiny appearance.

### Band Gap Overlapping VIS Spectrum
- Occurs when light is selectively absorbed within the visible light range due to overlapping band gaps.
- Chemical impurities can introduce additional electron energy levels.
- These impurities enable selective absorption, resulting in color.

### Examples of Gemstones with specific causes of colour
- Sapphire's blue color is due to Fe2+ and Ti4+ impurities.
- Amethyst's purple color is caused by Fe2+, with a yellow component from Fe3+.
- Yellow beryl contains O2- and Fe3+.
- Green diamonds can be naturally or artificially colored by radiation. 
- The GR1 absorption line is at 741nm.

### Quartz
- Irradiation over long periods can eject electrons, causing absorption and black coloration.
- This coloration can be reversed with heating.

### Purple Fluorite
- In purple fluorite, a fluorine ion is missing, leading to an electron filling the vacancy.
- This electron absorbs yellow light, resulting in the perception of purple.

### Yellow Diamond (Type 1AAB)
- A spectral line may be observed at 415nm.
- Can exhibit blue fluorescence under long-wave UV light, creating an apparent blue body color if strong enough.

### Pink, Mauve, and Brown Diamonds
- Believed to be colored by plastic deformation and can be treated with HPHT.

### Yellow Diamonds
- When nitrogen replaces carbon in the diamond lattice, it introduces a 'donor' level.
- Electrons move to the conductance band, interact with light energy, and absorb violet photons.
- This results in a 'canary' yellow color in proportion to the concentration of nitrogen atoms.

### Blue Diamonds
- When carbon is replaced by boron with only three outer electrons, there is a deficiency of one electron and boron forms acceptor level.
- Energy is absorbed near infrared, while blue and violet energy levels are transmitted.

### Electrical Current in Diamonds
- Boron's position near the top of the valence band allows for thermal excitation of electrons.
- This creates 'holes' in the valence band, enabling electrical current flow.
- Diamonds containing boron as a major impurity conduct electricity.
- Higher boron concentration equates to greater electrical conductivity and blue color.
- Non-conducting blue diamonds can be created by color centers.

### Type Ia Diamonds
- The majority of natural diamonds are Type Ia, known as the Cape series.
- Believed to have formed with nitrogen dispersed as individual atoms, then aggregated over time due to heat and pressure.
- Most have aggregates of three nitrogen atoms around a vacancy.
- This creates a color center that absorbs energy in the blue region, resulting in a yellow tint.
- Plastic deformation can occur, leading to brown, pink, or red coloration with absorption at 503nm.
- Testing involves observing blue fluorescence under UV light, which can be masked by body color.
- Sharp absorption line at 415nm, may be accompanied by up to five other weak bands in the violet.

### Type Ib Diamonds
- A deep golden yellow may be observed, traditionally called 'Canary' yellow.
- Believed to have formed with nitrogen dispersed as single nitrogen atoms.
- Nitrogen occurs as isolated atoms replacing carbon atoms randomly.
- The surplus electron in bonding leads to the absorption of blue light, resulting in a yellow color.
- Natural type 1b diamonds are rare.
-  Many synthetic diamonds are type Ib because their dispersed nitrogen atoms haven't had time to aggregate.
- Testing involves absorption in the blue region, sometimes with peaks at 503nm and 637nm.

### Type IIa
- Almost pure, containing negligible amounts of nitrogen.
- Appear colorless if no pervading impurities or imperfections are present.

### Type IIb
- Contain boron atoms as a major impurity.
- Boron atoms have deficiency in energy, is absorbed in the longer visible wavelengths
- This results in a blue color.
- The Hope diamond is a famous example.
- Exhibits unusual electrical properties due to boron.
- Inert under LWUV, with red fluorescence in SWUV.

### Green Diamonds
- Vacancies caused by irradiation lead to absorption at 741nm (GR1).
- Brown, greenish-yellow, and green stones show green fluorescence, and may show a narrow band at 503nm + weaker bands in the blue and green.

### Black Diamonds
- A large number of dark, opaque inclusions cause black color.

### Pink and Red Diamonds
-  May be caused by plastic deformation and found at 560nm.

### Brown Diamonds
-  Caused by plastic deformation and found at 560nm.

### Diamond Synthesis
- Laboratory techniques duplicate the conditions of natural diamond formation.
- Two main processes are used: HPHT and CVD.

### Diamond Synthesis: HPHT
- Creates synthetic diamonds under high temperature and pressure.
- Replicates the conditions of mantle-grown diamonds.
- BARS and Belt processes differ in how pressure is applied to the reaction cell.
- Reaction cell is kept at high temperatures (1350-1600°C) and pressures (50-80 kbars).
- Carbon feed material (graphite or diamond powder) is used.
- A catalyst (flux) like nickel, iron, or cobalt assists in melting the carbon.
- Temperature at the base of the reaction cell is slightly lower.
- Encourages crystal growth on a seed crystal.

### Diamond Synthesis: CVD
- No sublimation process is needed as ingredients come in gaseous form.
- A combination of methane, hydrogen, oxygen, and argon is passed through a microwave.
- Atoms of carbon are deposited on seed plates to build a diamond crystal.
- Developments using a seed crystal of diamond are suitable for manufacturing.
- Increasingly abundant due to quicker growth rates and lower cost.

### Distinguishing Features of Synthetic Diamonds
- Synthetic diamonds often contain metallic flux and dust inclusions.
- Hourglass color zoning.
- Metal catalyst may be discovered as inclusions.
- Distinctive crystal habit, forming combined shapes like truncated cubes or octahedrons.
-  UV fluorescence is stronger in short wave than long wave.
- Typically shows yellow or yellow-green fluorescence.
- Phosphorescence may occur after shortwave UV.
- Note, only approximately 1/3 fluoresce.
- Most natural diamonds have an absorption band at 415nm; this is not found in synthetic diamonds.
- Exhibits magnetism - some attracted to high powered magnet.
- Detects the 415nm absorption seen in most diamonds.

### Diamond Mining
- Diamonds found in kimberlite and lamproite, or in river/beach gravels after erosion.
- 100 tonnes of ore contain only 1g (5ct) of gem-quality diamonds.
- Diamonds may be laser drilled to reduce visual impact of dark inclusions.
- Exceptional hardness, brilliance, fire, luster sets them apart
- Thermal tests separate diamonds from simulants.
- Fluorescence: ranges from inert to strong.
- Natural diamonds most fluoresce to LWUV light, bluish-white in hue.
- Visual clarity assessed from Flawless (FL) to Imperfect (I)
- Clarity: number, size, position, nature, brightness of inclusions
- Cut: style, symmetry, proportions and polish is tested

### Pear Lustre
- Pearl lustre is unique and complicated, also known as orient
- It is caused by the reflection of light and iridescence of light.
- Pearl nacre consists of organic-rich layer and aragonite
- May be damaged by parasites, disease or predator attack from pests.
- Created using tissue nucleation process using a section of the mantle.

### Imitation Pearls
- With coated surfaces has layers or chipping of the coating around drill holes.
- Its surface may appear granular when compared with overlapping pearl structure.
- Pearl quality is assessed in reference to its luminescence, color, shape, surface and size.

### Marine Pearl Creation
- The process involves pearl farms and the creation of oysters by pearl farmers.
- The growing process is from for 2-3 years with constant care until a nacre forms.
- Akoya pearls produce mostly smaller, white pearls
- Oysters grow to 10cm big.
- Black-lipped South Sea pearl oysters produce black and grey iridescent pearls
- Silver and Gold lipped South Sea pearl oysters produce larger white and golden pearls.
- Mabe oyster produce Mabe pearls

### Freshwater Pearls Creation
- The pearl mussels are harvested and small pieces of the mussel are placed, then suspended in lakes.
- For smaller pearls, up to 30 can be grown at the same time.
- Larger pearls product less.
- Freshwater pearls turn grey with irradiation.

### Quality Factors In Gemstones
- The most valuable rubies have a good rich red color which are often free from inclusions.
- Best quality emeralds have rich soft green which is often higher in value.
-  Zambian emeralds are said to have a cooler bluish-green colour.

### Types Of Inclusions
- Existed as a complete material prior to being enveloped referred to as Protogenetic
- Formed at the same time as the host gem and was therefore Brough into existence by the formation gemstone referred to Syngenetic

### Laboratory Testing: Energy Dispersive X-Ray Fluorescence Spectroscopy (EDXRF)
- Detects trace element impurities
- Distinguishes between natural and synthetic materials
-  Helps identify locality and origin.

### Laboratory Testing: Scanning Electron Microscope (SEM)
- Used to learn about the surface of the gemstone in extraordinary detail prior to treatment.
- Uses a beam of electrons is projected onto material's surface

### Laboratory Testing: Fourier Transform Infrared Spectroscopy
- Useful to detect treatments.
- Can identify diamond types and separate natural and synthetic gem materials.

### Laboratory Testing: Laser Raman Spectroscopy (LRS)
- Identifies gemstone and inclusion
- Can detect a  wide range of treatments
- Insight on origin and locality, and can identify evidence of HPHT treatment of diamond

### Laboratory Testing: Ultraviolet-Visible-Near-Infrared Spectroscope (UV-Vis-NIR)
- determines the origin.
- identifies HPHT treatment in diamonds
- Identify synthesis in emerald (based on absorption) and corundum (based on presence of iron)

### Gemological Testing: Spectroscope
- Composed of prisms (formed of two types of glass) which produce a spectrum by refraction and dispersion
- Fine ruled grating which produces a spectrum by diffraction, light from the split is focused onto the grating

### Gemological Testing: Refractometer
- RI of material is related to its critical angle
- Distant Vision technique is commonly used.
- Stone on table with fluid, look through eyepiece
- Bright Line Technique creates rainbows

### Gemological Testing: Polariscope
- Two filters: 'Polariser' and 'the Analyser'
- Isotropic materials remain dark while Anisotropic show different light directions
- Used the detect UV light

### Gemological Testing: Dichroscope
- Calicte or London
- Can distinguish between UV light
- Alexandrite shows red, orange and green
- Iolite shows violet blue , pale blue and pale yellow to colourless
- Tanzanite shows violet blue , pale blue and yellow green

#### Treatment
- Can change or improve its appearance
- Some Treatments are bleaching to improve color

### Treatment: Bleaching
- Effective in porous gems.
- Identification difficult unless colour changes drastically

### Treatment: Dyeing
- Introduced to permeable materials to alter or produce colour
- Can easily be removed
- Observable in cracks and fissures.

### Treatment: Coating
- Applied to change color
- Non permanent
- May be observable with evidence of wear or spectrum is easily removable.
- Foiling to give gem brilliance and is best used on loose stones.

### Treatment: Impregnation
- Apply wax or polymer resin to the surface of porous stone to toughness and is best on turquoise and jadeite.

### Treatment: Healing/Heating
- Most pink topaz on the market is heat treated to create pink/purple-red topaz and is most commonly done with clear amber

### Treatment:Fracture filling
- Used to filling on fractures to avoid cracks from appearing