Tanzanite
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.
Commonly Confused With
Commonly confused with: Sapphire, Iolite, synthetic_corundum, glass, Zircon.
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.
About Tanzanite
The blue to violet variety of zoisite, known for its striking trichroism.
Identifying a tanzanite? GemID walks through these tests in order — RI, SG, fluorescence, and more.
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