Tanzanite
JewelryWiki Editorial·7 sources·Reviewed 2026-08-04
Fails on two of the three durability axes at once: low toughness, and colour that can change under prolonged light.
Properties
| Property | Value | Authority |
|---|---|---|
| Mohs hardness | 6½–7 [2] | — |
| Toughness | Low. GIA groups tanzanite with opal as "not-so-tough" gems needing protection from accidental blows. [1] | — |
| Stability | Poor under prolonged light. GIA lists tanzanite with kunzite and topaz as gems that can fade or change colour from prolonged exposure to light. [1] | — |
| Refractive index | 1.691–1.700 [2] | — |
| Specific gravity | 3.35 [2] | — |
| Chemical formula | Ca2Al3(SiO4)(Si2O7)O(OH) Read from the species entry, Zoisite. A variety shares its species’ formula — unlike hardness or density, which vary with composition and are not inherited here. [7] | — |
| Crystal system | Orthorhombic [4] Read from the species article, Zoisite. | — |
| Common treatments | Routinely heated. Zoisite, the mineral species, is often heated at low temperatures to remove a brownish component and produce the stronger purplish-blue the stone is bought for — GIA illustrates the same material brownish as mined and blue after heating. The treatment is considered durable and permanent under normal handling. [6] | — |
| Suitability for daily wear | Occasional wear. Earrings and necklaces preferred; reserve rings and bracelets for special occasions. [1] | — |
| Care cautions | NEVER use an ultrasonic cleaner — GIA names this among the gems susceptible to heat and temperature change, which should be kept out of an ultrasonic REGARDLESS of whether they have been treated. Use warm soapy water or a solution formulated for delicate gems. [5] | — |
| Birthstone month | December — added as an official December birthstone in 2002 by AGTA and Jewelers of America, recognising the commercial prominence of a species discovered only in 1967. Shares December with turquoise and blue zircon. [3] | — |
Sources
- GIA — More than the Mohs Scale: Understanding Gem Durability — Gemological Institute of America (GIA 4Cs, "More than the Mohs Scale" (14 March 2016))
- International Gem Society — reference charts (Mohs hardness, specific gravity, refractive index) — International Gem Society (IGS reference charts)
- Jewelers of America — Birthstones — Jewelers of America (Jewelers of America, Birthstones)
- Wikipedia — mineral article infoboxes (crystal system) — Wikipedia contributors (Wikipedia mineral infoboxes)
- GIA — Ultrasonic Cleaners for Gems: What You Need to Know — Gemological Institute of America (GIA 4Cs, Ultrasonic Cleaners for Gems)
- GIA — An Introduction to Gem Treatments — Gemological Institute of America (GIA, An Introduction to Gem Treatments (Robert Weldon))
- Handbook of Mineralogy — Mineral Data Publishing / Mineralogical Society of America (Handbook of Mineralogy, Mineral Data Publishing)