Stone Info_ Garnet & Spinel .pdf

*****Garnet & Spinel ***** LARGEST CONSUMERS : Tsavorite: Demand in Asia Pyrope: Marketed as an "American" gem stone Spinel: 5-ct. red spinel might sell for a fraction of anequivalent-quality ruby, Bright, very intense pink spinels command high prices, amongst collectors Mogok flatten twinning spine...

*****Garnet & Spinel ***** LARGEST CONSUMERS : Tsavorite: Demand in Asia Pyrope: Marketed as an "American" gem stone Spinel: 5-ct. red spinel might sell for a fraction of anequivalent-quality ruby, Bright, very intense pink spinels command high prices, amongst collectors Mogok flatten twinning spinel with “Star of David” are in demand in pink & other colors MOHS SCALE: Demantoid: 6.5 VARIETY/SPECIES/GROUP: Garnets: 20 garnet species, with only 5 commercially imported as gems, Many garnets are mixtures of two or more garnet species, demantoid is variety of andradite, tsavorite and hessonite are varieties of grossular, Rhodolite is a mixture of pyrope and almandine COLOR/HUE RANGE/VALUE: GarnetTsavorite: yellowish green through bluish green (if they are weakly saturated are comsidered grossular garnet) most prized colors are pure green or green w/ slight bit of blue Demantoid: yellowish or brownish green hue to a very rare, intense green, best color moderately strong to strongly saturated green of medium to medium-dark tone, with a higher dispersion value than diamond. -“topazolite”- pure yellow andradite Spessartine: pale yellowish orange through bright orange to a dark brownish red, mos t favored color highly saturated, slightly reddish orange hue with medium to medium-dark tone Malaya: pinkish orange mix of pyrope, spessartine & small amount of almandine,"peach" or "cinnamon" Hessonite: orange to cinnamon-colored variety of grossular garnet with a brownish hue Rhodolite: range from a dark, purplish raspberry red to a light reddish purple. In the best gems, the tone is medium to medium dark, with strong saturation, often inclusion free Pyrope: range of orange-red to slightly purplish red to strongly reddish purple hues Almandine: range of orange-red to slightly purplish red to strongly reddish purple hues (same as Pyrope) Pyrope-Almandine Mix (not rhodolite): other types of this mix can be dark-toned brownish orange to brownish red to purplish red hue, with low saturation. Spinel: orange to intense “stoplight” red, vibrant pink, and all shades among purple, blue, and violet through to bluish green, Pure red to slightly purplish red hues with medium to medium-dark tone, vivid saturation, and high brightness are considered the finest of all, red is the most desired followed by fine cobalt CHEMICAL CO MPOSTION: Garnet: basic composition is silicon, oxygen, and aluminum, with is omorphous replacement being possible, Tsavorite: trace element of vanadium Demantoid: trace element of chromium and iron Spessartine: Trace elements of manganese and iron Malaya: Trace elements of manganese and iron Hessonite: Trace elements of manganese and iron Pyrope: top stones have trace elements of chromium, iron pyrope has less vibrant hues Spinel: Intense reds and pinks are caused by traces of chromium, Orange & purple stones is a mixture of iron and chromium, Orange and purple stones owe their color to a mixture of iron and chromium has trace elements of iron, iron-to-iron charge transfer produces a bluish green to greenish blue, rare vibrant “cobalt” blue stones from Sri Lanka, Madagascar, and Pakistan have cobalt and iron FORMING SOURCE: Garnets: metamorphic rocks and pegmatite veins, Demantoid: volcanic and metamorphic rocks Spessartine: pegmatites and metamorphic rocks Rhodolite: Alluvial deposits and metamorphic rock Almandine: metamorphic rocks like schis Spinel: in marble and alluvial deposits CRYSTAL STRUCTURE: Garnets: all have a cubic crystal system, with most common habit being dodecahedron their chemical compositions vary, though Tsavorite: rarely well formed crystal but more irregular nodules called "potatoes" or "pods", rough is typically fractured, and so it is calibrated into 1ct or smaller Spinel: Cubic crystal system, crystal habit is an octahedron, can form flattened twin crystals, singly refractive CLARIETY CHARACTERISTIC: Garnet & Spinel are singly refractive Tsavorite: extinction in darker stone Demantoid: curving, radiating bundles of fibers known as “horsetails.” (fine silky strands), these inlcusions can increase the value of a stone Hessonite: Most hessonite is too included to be used in jewelry, crystal inclusions—usually apatite, diopside, and zircon, these inclusions cause an instantly recognizable “heat wave” or “scotch-in-water” identifying them as hessonite COMMON CUTS: Demantoid: rarely found over 3ct, and typically used in pave settings Tsavorite: calibrated squares, rounds, and ovals under one carat, 2- to 4-mm rounds and ovals up to 6×4 mm are also popular,7×5 mm and larger are more rare Spessartine: can be cut large without loosing brilliance or color Rhodolite: good for being calibrated, prices rise steeply at 10ct and above, often cut into free form cuts by lapidare designers Pyrope: small red brilliant-cut or rose-cut pyropes Almandine: cut a cabochon with a curved undercut bottom a technique used to lighten stones (know as a "hollowback")reflective foil backings to improve their dark appearance, and shallow facet proportions Pyrope-Almandine Mix (not rhodolite):calibrated shapes, including rounds, ovals, octagons, squares, hearts, pear shapes, and trillions, larger the carats darker the tone and are often cut to shallow proportions Spinel: due to scarcity most rough is cut in free sizes instead of calibrated cuts, available calibrated stones tend to be mixed cut ovals POSSIBLE PHENOMENA: Color Change Garnet: strong red under one light source and green, or even blue, under another are a special type of pyrope-spessartine that contains vanadium, -another type of pyrope and spessartine have a color change from red to orange, orange to pink, pink to purple, or orange to yellow Star Garnet: a mix of almandine or rhodolite, star garnets are filled with rutile silk needles that cause asterism, 4 rays are common but 6 rays is also possible and sometimes they can have both in the same stone Spinel: Color Change and Start exist and is limited for collectors TRADES NAMES: pyrope-almandine garnet: "African” and “Mozambique” for dark-toned orange-red to brownish red and "Thai" garnets are darker in tone then African Demantoid: " Gold Green" is Greenish yellow, yellowish green stones Pyrope: "anthill garnets " from Arizona and Utah Spinel: "Balas Ruby" In ancient times, the mines of Asia yielded exceptionally large spinel crystals known as Balas rubies, most famous example is “Black Prince’s Ruby” which is a part of the English crown jewels CUTTING/TRADING CENTER: Tsavorite: calibrated stones are sent to China, Thailand, India, and Sri Lanka, Large, fine-quality rough are sent to Germany's Idar-Oberstein Spessartine: China, Thailand, and India for cutting, prices for large, good-quality stones are high COUNTRIES OF ORIGIN: Tsavorite: Pakistan, most tsavorite comes from the East African countries of Tanzania, Kenya, and Madagascar, Scorpion mine is a famous tsavorite source in Tsavo Park, Kenya until it was shut down Demantoid: the stone long association with Russia and Russian royalty, and Russia is a source for very highquality demantoid, stone form the Ural Mountains are well known, Usakos in Central Namibia in Africa, Madagascar, Iran, Italy, Greece, and Mexico Spessartine: famous mine Little Three mine in Ramona, California, Rutherford mines in Virginia, w was open for a limited time, more recently Madagascar, Namibia, Tanzania, Zambia, India, Pakistan, Sri Lanka, Brazil, and especially Nigeria, Brazil states of Minas Gerais and Rio Grande do Norte produce large carats Malaya: alluvial deposits in Tanzania’s Umba River Valley, Kenya, Madagascar, and Sri Lanka Hessonite: India, Sri Lanka, Brazil, Afghanistan, and both Kenya and Tanzania, Mexico provides good-quality strong orange Rhodolite: first discovered in North Caroline, US, in recent years Eastern Africa has been the most important source, Tanzania, Umba River, Sri Lanka, India’s Odisha State, and Madagascar Pyrope: Podsedice mine in the Bohemian Hills of the Czech Republic Almandine: Myanmar, Thailand, Brazil, Tanzania, and Mozambique, found in impressively large sizes, often exceeding 75 to 100 mm Color Change Garnet: Alexandrite-like color-change garnets from Africa and Sri Lanka is pyrope and spessartine mix, another type is found in Tanzania’s Umba Valley and Madagascar (see phenomena) Star Garnet: India, Sri Lanka, and the US state of Idaho Spinel: neon pink spinels from Man Sin, Myanmar, Sri Lanka and Afghanistan, the Pamir Mountains in Tajikistan, Myanmar’s Mogok region produces fine red and pink spinel, Tanzania’s Morogoro region, Ilakaka area of southern Madagascar have purplish violet to bluish violet and purple to reddish purple stones COMMON TREATMENTS: Spinel: most are considered untreated, but pink to red spinels might be heated to improve their appearance, also a cobalt diffusion treatment that creates fine blue color in spinel MAIN SYNTHETIC PROCESS: no synthetic or imitations red garnets have been produced as the actual stones are readily available Spinel: Russian-made flux-grown synthetic spinel, Synthetic flame-fusion spinel is also manufactured into triplets, COMMON IMITATION MATERIAL: Garnet: Colored YAG or CZ might imitate the more valuable green garnets—like tsavorite and demantoid—as these materials are of very similar appearance