Tanzanite Simulants and Treatments
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Tanzanite: Simulants and Treatments by Kaylan Khourie, FGA General Description Tanzanite is best known as the transparent Blue-Violet variety of Zoisite. It has, up until the date of writing, only been mined in the Manyara Region of Tanzania (hence the name ‘Tanzanite’), near the Merelani Hills where it was discovered in 1968. Due to its beauty and relative rarity (as it is only mined in one location), it has become very desirable in the market and had been added to the list of birthstones for December (along with Turquoise and Zircon) by the American Gem Trade Association (AGTA) in October 2002. The finest quality gems display a deep Violet- Blue colour that, when cut correctly, causes both of the dominant pleochroic hues to coalesce. Although the term ‘Fancy Tanzanite’ has become increasingly popular to satisfy the demands of the market - this includes any other colour of A Large Fine Quality Tanzanite transparent Zoisite, such as: Pink, Green and Yellow – this article will not be going into too much detail regarding ‘Fancy Tanzanite’. Tanzanite has a Mohs hardness of 6-7 which makes it vulnerable to scratches and abrasions. It has a perfect prismatic cleavage so it is susceptible to breaks. It is very sensitive to ultrasonic cleaning so one should rather use a soft brush and some warm water (sometimes with some dishwashing liquid) to scrub off any grime. Graphics and text subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Tanzanite Simulants Simulants are gemstones that are either: grown in a laboratory without a natural counterpart or a natural/laboratory-grown gemstone that is imitating another gemstone without the same physical, chemical and/or optical properties. Because Tanzanite is so desirable, many simulants have been used to imitate its appearance as a cheaper alternative (both natural and laboratory-grown). To date, there have been no successful attempts to synthesize gem-quality Tanzanite in a laboratory. Unfortunately, with this come many attempts to deceive the consumer into paying for a simulant as if it were a natural Tanzanite. Gemmological laboratories have offered gemstone identification services to protect the consumer from this sort of misrepresentation. The author has come across a range of imitations that have been presented as Tanzanite – below are the most convincing imitations and their characteristics as assessed, some of these characteristics – such as pleochroism – can be used as effective ways to separate Tanzanite from its simulants: • Cubic Zirconia (CZ) Usually Purple-Violet in colour. Its high specific gravity contributes to its significant heft. Its strong birefringence dispersion makes it fairly easy to distinguish it from Tanzanite. Pleochroism: None - Isotropic • Laboratory-grown Spinel (Flame-fusion method) Usually deep Blue in colour. Its specific gravity is almost identical to that of Tanzanite, so it is not possible to separate it from Tanzanite using heft. Pleochroism: None - Isotropic Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. • Kyanite Usually Blue in colour. Its differential hardness (from 4 in one direction to 7 in another direction) may affect the quality of polish of the faceted gem, whereas great care is usually taken to ensure the finished Tanzanite gem is of the best possible polish and symmetry. Kyanite’s specific gravity is close to that of Tanzanite so it will have a similar heft. It often has very distinct colour-zoning which distinguishes it from Tanzanite. Unlike Tanzanite, it is almost never found without significant inclusions. Pleochroism: Moderate (Near Colourless/ Dark Blue-Violet/ Blue) • Iolite Usually bluish Violet. Its lower specific gravity (and therefore lower heft), trichroic colours and generally less intense colour can help distinguish it from Tanzanite. Pleochroism: Moderate (Colourless to Yellow/ Blue/ Dark Blue-Violet) • Sapphire Usually violetish Blue, the higher specific gravity may aid differentiation using heft. It is part of the Corundum family and therefore has a much higher hardness that helps it resist the scratching and abrasions that Tanzanite is prone to. Often displays significant colour-zoning (but it is usually cut in an attempt to try and hide it). Natural Sapphire (Left) often has distinct linear or angular colour banding. Flame- fusion laboratory-grown Sapphire (Right) usually has distinct curved colour banding. Pleochroism: Weak (violetish Blue/Blue) Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. • Synthetic Forsterite (also known as ‘Tanzanion’) Created specifically to imitate Tanzanite, it has a specific gravity almost identical to Tanzanite. It is grown using the Czochralski method and often contains many tiny bubbles that gives its true identity away. It displays strong doubling which is also uncommon in Tanzanite. Pleochroism: Distinct (Purple to Blue/Yellow/bluish Green) • Cobalt-doped Glass Usually a deep violetish Blue. The colour appearance and specific gravity is achieved by doping the glass with Cobalt amongst other trace elements and surprisingly makes this type of glass the most convincing imitation of Tanzanite. Pleochroism: N/A - Amorphous • Dyed Glass in ‘tar’ as rough Many pieces of glass covered in tar have also been encountered. These are intended to simulate rough Tanzanite and other gemstones; simply dyed in order to imitate the specific gemstone, i.e. Red for Ruby, Green for Emerald and of course Blue-Violet for Tanzanite. This is clearly identified as dyed glass by its large conchoidal fractures that have the colour concentrated inside them from the dye. Pleochroism: N/A – Coloured by Dye Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Comparisons between Tanzanite and each Simulant Top to Bottom, Left to Right: Row 1: Cobalt- doped Glass Row 2: CZ; CZ; CZ; Lab-grown Spinel Row 3: Lab-grown Sapphires Row 4: Iolite; Iolite; Natural Sapphire; Kyanite Row 5: Tanzanites of different saturations Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Specific Gravity Gemstone Common Colour Colour Cause4, 5 Refractive Index Polariscope Hardness (Average) V4+ in octahedral coordination; with Tanzanite (Zoisite) Blue-Violet 1.691 - 1.7042 3.302 DR – Biaxial + 6 - 7 V3+ in octahedral coordination1 Co₂O₃; MnO₂; Nd₂O₃ Cubic Zirconia (CZ) Purple-Violet 2.152 5.802 SR/ADR 8 - 8.5 doping Co2+ and Fe2+ in Laboratory-grown ADR – “Tabby Deep Blue tetrahedral 1.732 3.642 8 Spinel Extinction” coordination Fe2+ - O - Ti4+ charge Kyanite Blue 1.716 - 1.7312 3.622 DR – Biaxial + 4 - 7 transfer Fe2+ -> Fe3+ charge Iolite (Cordierite) Violet 1.542 - 1.5512 2.612 DR – Biaxial +/- 7 - 7.5 transfer 6*Intervalence Sapphire charge transfer of Blue/violetish Blue 1.762 - 1.7702 4.002 DR – Uniaxial - 9 (Corundum) Fe2+ -> Ti4+ or Fe2+ -> Fe3+ Laboratory-grown Violet-Blue Cr3+ and Cr4+ doping3 1.636 - 1.6691 3.153 DR – Biaxial +3 6.5 - 7 Forsterite (Olivine) CoO2 and/or MnO2 Varies. Commonly Varies. Commonly ADR – “Writhing Glass Deep violetish-Blue 5 - 6 doping 1.52, 1.67, 1.75 2.80, 3.30, 4.10 Snakes” 1Zancanella, V. 2004. Tanzanite: All about one of the most fascinating gemstones: The True Story. Naturalis Historia. 2Matlins, A.L. & Bonnano, A.C. 1995. Gem Identification Made Easy. London: St Edmundsbury Press Ltd. 3Nassau, K. 1994. Synthetic Forsterite and Synthetic Peridot. Gems and Gemology. Summer 1994 pp 102 – 108. 4Fritsch, E. & Rossman, G.R. 1998. An update on color in gems. Part 2: Colors involving multiple atoms and color centers. Gems and Gemology. Spring ed. 5Nassau, K. 1994. Synthetic Forsterite and Synthetic Peridot. Gems and Gemology. Spring ed. 6Dubinsky, E.V. 2020. A Quantitative Description of the Causes of Color in Corundum. Gems and Gemology. Summer ed. Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Polariscope Observations Uniaxial Optic interference figure using a Polariscope “Tabby Extinction” Strain and Conoscope using a Polariscope Biaxial Optic interference “Writhing Snakes” strain figure using a Polariscope using a Polariscope and Conoscope Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Inclusions The most common feature in Glass Curved growth/colour banding is often are bubbles and swirl marks Lab-grown Forsterite will have observed in flame-fusion Lab-grown strong doubling and often has Sapphires an abundance of bubbles Kyanite often exhibits distinct Natural Sapphires have a large range of Fine Tanzanite seldomly has any angular growth zoning possible inclusions – this is an inclusions but growth tubes, healed iridescent thin-film fractures and liquid planes can be seen Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Vis-NIR Comparisons Cubic Zirconia vs Natural Tanzanite Purple – Tanzanite Dark Orange – Cubic Zirconia Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Laboratory-grown Spinel vs Natural Tanzanite Purple – Tanzanite Green – Laboratory-grown Spinel Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Kyanite vs Natural Tanzanite Purple – Tanzanite Blue – Kyanite Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Iolite vs Natural Tanzanite Purple – Tanzanite Blue-Green – Iolite Graphics and text are subject to copyright