Tanzanite
Last updated: April 2026
The blue to violet variety of zoisite, known for its striking trichroism.
Physical & Optical Properties
RI Range1.691–1.700
SG Range3.35–3.38
SG Typical3.35
Hardness (Mohs)6–7
Crystal SystemOrthorhombic
Optic CharacterDR Biaxial (+)
Birefringence0.009
Dispersion0.030
Fluorescence LWWeak Red Brown
Fluorescence SWInert
Chelsea FilterInert
PleochroismStrong Trichroic
ColorsBlue Violet, Purple
SpeciesZoisite
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 1.762–1.778; SG 4.00; dichroic not trichroic; much harder (9 Mohs).
- Iolite: Iolite: biaxial negative; RI 1.542–1.551; SG 2.58–2.66; trichroism shows pale yellow, not red-violet.
- Synthetic blue-violet corundum: Uniaxial negative; RI 1.762–1.778; 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)
Price Context
Natural — low ($/ct)$400
Natural — high ($/ct)$3,000
NotePer carat; fine vivid violet-blue AAA grade; all commercial tanzanite is heat-treated
Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.
Related Comparisons
Measurement Guides
Identifying a tanzanite? GemID walks through these tests in order — RI, SG, fluorescence, and more.
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