Tanzanite

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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

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

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 1.54–1.56; 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.

Related Comparisons

Measurement Guides

Identifying a tanzanite? GemID walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Tanzanite is one of the 130 species in GemID’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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