Tanzanite
JewelryWiki Editorial·4 sources·Reviewed 2026-07-31
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] | — |
| Crystal system | Orthorhombic [4] Read from the species article, Zoisite. | — |
| Suitability for daily wear | Occasional wear. Earrings and necklaces preferred; reserve rings and bracelets for special occasions. [1] | — |
| 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)