Tanzanite
Last updated: April 2026
The blue to violet variety of zoisite, known for its striking trichroism.
Tanzanite has a specific gravity of 3.35 and a refractive index of 1.691–1.700, with a Mohs hardness of 6.0–7.0. It is biaxial +.
Physical & Optical Properties
Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.
Through the refractometer
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Related: Zoisite Varieties
Key Differentiators
- Extreme trichroism in heated (commercial) material: a-axis blue-violet, b-axis purple, c-axis green-yellow; unheated material shows brownish-red on one axis
- Relatively low hardness (6–7 Mohs) — tanzanite requires careful wear
- Found only in Tanzania — single source provenance
Common Simulants
- Blue sapphire: Sapphire: uniaxial negative; RI 1.762–1.770; SG 4.00; dichroic not trichroic; much harder (9 Mohs).
- Iolite: Iolite: biaxial negative; RI 1.54–1.56; SG 2.57–2.66; trichroism shows pale yellow, not red-violet.
- Synthetic blue-violet corundum: Uniaxial negative; RI 1.762–1.770; SG 4.00; dichroic; no tanzanite-characteristic trichroism; check inclusions.
- Blue-violet glass: Isotropic; no trichroism; gas bubbles under loupe; conchoidal fracture.
Treatments
- Heat Treatment (to blue-violet)
- Cobalt Surface Coating (rare)
Related Comparisons
Measurement Guides
Identifying a tanzanite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Tanzanite is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →