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Home , Gemology, Pleochroism, Tanzanite

: Simulants and Treatments by Kaylan Khourie, FGA General Description Tanzanite is best known as the transparent Blue-Violet variety of . It has, up until the date of writing, only been mined in the Manyara Region of (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 for December (along with and ) 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 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 that are either: grown in a laboratory without a natural counterpart or a natural/laboratory-grown 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 – can be used as effective ways to separate Tanzanite from its simulants:

• (CZ) Usually Purple-Violet in colour. Its high specific gravity contributes to its significant heft. Its strong makes it fairly easy to distinguish it from Tanzanite. Pleochroism: None - Isotropic

• Laboratory-grown (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. • 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)

• 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 (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)

• -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 , Green for 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 Row 4: Iolite; Iolite; Natural Sapphire; Kyanite Row 5: of different saturations

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Specific Gravity Gemstone Common Colour Colour Cause4, 5 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 () 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 ()

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 . Gems and . 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 – Kaylan Khourie, FGA – unless specified otherwise. Sapphire vs Natural Tanzanite

Purple – Tanzanite

Maroon – Flame-fusion Laboratory- grown Blue Sapphire

Red – Natural Basaltic Blue Sapphire

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Laboratory-grown Forsterite vs Natural Tanzanite

Purple – Tanzanite

Cyan – Laboratory-grown Forsterite

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Glass vs Natural Tanzanite

Purple - Tanzanite Purple – Tanzanite Cyan - Laboratory-grown Forsterite Black – bluish Violet Glass

Olive – Vivid Blue-Violet Glass

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. FTIR Comparisons Cubic Zirconia vs Natural Tanzanite

Purple – Tanzanite

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 – Kaylan Khourie, FGA – unless specified otherwise. Sapphire vs Natural Tanzanite

Purple – Tanzanite

Maroon – Flame-fusion Laboratory- grown Blue Sapphire

Red – Natural Basaltic (heated) Blue Sapphire

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Laboratory-grown Forsterite vs Natural Tanzanite

Purple – Tanzanite

Cyan – Laboratory-grown Forsterite

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Glass vs Natural Tanzanite

Purple – Tanzanite

Black – bluish Violet Glass

Olive – Vivid Blue-Violet Glass

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Tanzanite Treatments “Treatments” are artificial processes that gemstones are subjected to in order to alter or enhance their properties: usually their colour appearance (such as irradiated ) and clarity appearance (such as oiled Emerald), but can also be used to alter their transparency (heat-treated Ruby), strengthen their durability (impregnated ) and create phenomena (diffused Star Sapphire). Disclosure of treatments is very important to conserve consumer confidence. The consumer needs to understand whether or not their gemstone has undergone any artificial processes to alter their properties and whether that process has any effect on the current market value, the future market value and/or the durability of the gemstones. At the date of writing, the only treatments that Tanzanite is subjected to are:

• Heating Almost all the Tanzanite available in the market have been gently heated from a ‘khaki’ colour to the deep Blue-Violet colour that it is known for. There is debate regarding whether this is considered a treatment because the change occurs at low temperatures and for a short duration (relative to most gemstone heat treatments) and the stones are often “naturally heated” by heat vents while they are still in the ground which changes their colour. Nassau (1984) reports a change of the Yellow-Green colour component to a deep Blue at 370°C for 2 hours. However, the author is of the opinion that if a gemstone has been subjected to any non-natural process – other than cleaning and the standard polishing process – it should be classed as “treated” or any of the similar industry- accepted terms ie: “Enhanced”, “Artificially Irradiated”, etc. There are cases – such as with the heat-treatment of Tanzanite – where it is not currently scientifically possible to identify whether the stone has definitely been heated “naturally” or artificially.

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. • Surface Coating In an attempt to improve the apparent colour appearance of stones with a lower saturation, some Tanzanites are coated with an extremely thin -based film on the pavilion of the stone. Although relatively uncommon in the modern era of treatments, it is quite easily detectable with a 10x loupe based on the clear peeling of the coating from general ‘wear and tear’.

• -filling Although rare, oil/resin-filled Tanzanite have been reported by R.W. Hughes (https://www.gia.edu/gems-gemology/winter-2020-gemnews-oiled-tanzanite). Ideally a filler with a refractive index close to that of Tanzanite (around 1.7) would be used to try and hide the fissure/s. However, paraffin oil or cedarwood oil (two of the most common Emerald fillers with RIs around 1.5) would be sufficient enough to hide the fractures and are used in many other non-emerald gemstones (such as , and corundum). Identification of such fillers relies upon microscopic observations and spectroscopic (such as FTIR or Raman) analyses. Glass-filling of Tanzanite has not yet been reported but one cannot rule it out as a possible treatment.

Conclusion When dealing with Tanzanite, it is always wise to apply standard gemmological techniques to make sure you know what you are dealing with. Keeping up to date with the latest news and publications – such as The Journal of Gemmology – is important to be aware of new gem finds, new laboratory- grown gems and new treatments. Many of the laboratory-grown gems and treatments can only be identified with advanced gemmological equipment, so if you are unsure of a particular gem, rather submit it to a capable gemmological laboratory. Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Sources Consulted Dubinsky, E.V. 2020. A Quantitative Description of the Causes of Color in Corundum. Gems and Gemology. Summer ed. Fritsch, 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. Liddicoat, R.T. Jr. 1977. Handbook of Gem Identification. Tenth Edition. Second Printing. United States of America: GIA. Matlins, A.L. & Bonnano, A.C. 1995. Gem Identification Made Easy. London: St Edmundsbury Press Ltd. Nassau, K. 1980. Gems Made by Man. Pennsylvania: Chilton Book Company. Nassau, K. 1984. Gemstone Enhancement. London: Butterworths. Nassau, K. 1994. Synthetic Forsterite and Synthetic Peridot. Gems and Gemology. Summer ed. O’Donohughe, M. 1983. Identifying man-made gems. London: NAG Press. Read, P.G. 1998. Gemmology. Oxford: Butterworth-Heinemann. Webster, J. 1975. Gems: Their Sources, Descriptions and Identification. Third Edition. London: Newnes-Butterworths. Zancanella, V. 2004. Tanzanite: All about one of the most fascinating gemstones. Italy: Natvralis Historia.

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise. Glossary

a) Amorphous: A gem that has no regular arrangement of atoms and therefore no structure.

b) Anisotropic: Exhibiting different properties/values when measured along crystal axes in different directions. Often evident as double refraction.

c) Birefringence: The strength of double refraction measured by the difference between the high and low refractive indices of a doubly refractive stone.

d) Dispersion: The separation of white light into its component colours.

e) FTIR graph: Fourier Transform Infrared Spectrometer Absorption Measurement.

f) Hardness: A material’s resistance to scratching or abrasion.

g) Heft is how heavy the gem ‘feels’ in relation to its size – it can be used as an estimation for a gem’s specific gravity when used in reference to another object of known relative specific gravity.

h) Isotropic: Exhibiting the same properties/values when measured along crystal axes in different directions. Will not exhibit double refraction.

i) Laboratory-grown: Also known as “Synthetic” or “Man-made”. Gemstones that are grown in a laboratory and have a natural counterpart with the same physical, chemical and optical properties.

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise.

j) Pleochroic/Trichroic/Dichroic: Derived from ‘Pleochroism’ – which means “many colours” and is caused by an anisotropic gem splitting light into two rays that each have a different selective absorption pattern and therefore a different colour. Observed with a dichroscope.

k) Refractive Index: The ratio of the velocity of light in air to its velocity in a gem. Measured using a refractometer. A useful value for the identification of gems.

l) Specific Gravity: The ratio of the weight of a gemstone to the weight of an equal volume of water. Measured using a scale with a specific gravity attachment. A useful value for the identification of gems.

m) Simulant: 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 optical properties.

n) Synthetic: see Laboratory-grown.

o) Treatment: An artificial (laboratory) process that a gem is subjected to in order to change or improve its colour, clarity, transparency, durability and/or to create phenomena.

p) Vis-NIR graph: Visible to Near-Infrared Absorption Measurement.

Graphics and text are subject to copyright – Kaylan Khourie, FGA – unless specified otherwise.