Sapphire vs Tanzanite: How to Tell Them Apart

Last updated: April 2026

Blue sapphire (corundum) and tanzanite (zoisite) can look strikingly similar in blue-violet hues. But their properties diverge sharply: sapphire is far harder and more durable, and the RI ranges do not overlap. Tanzanite's extraordinary trichroism — showing blue, violet, and burgundy in different directions — is the fastest visual clue.

Property Comparison

Property Sapphire (Corundum) Tanzanite (Zoisite) Diagnostic?
RI 1.762 – 1.770 1.691 – 1.700 Yes
SG 3.95 – 4.03 3.35 – 3.38 Yes
Hardness 9 6 – 7 Yes
Crystal System Trigonal Orthorhombic Yes
Optic Character DR U- (uniaxial negative) DR B+ (biaxial positive) No
Birefringence 0.008 0.009 No
Fluorescence (LW) Variable Weak red-brown No
Chelsea Filter Variable Inert No
Pleochroism Strong dichroic (violet-blue / greenish-blue) Strong trichroic (blue / violet / burgundy) Yes
Sapphire vs Tanzanite: where the ranges part Where the ranges part Sapphire and Tanzanite, each recorded property on an axis scaled to the pair Sapphire Tanzanite Refractive index separates no overlap — a gap of 0.062, wider than the ±0.010 a reading is trusted to 1.762–1.770 1.691–1.700 1.688 1.731 1.773 Specific gravity separates no overlap — a gap of 0.57, wider than the ±0.10 a reading is trusted to 3.95–4.03 3.35–3.38 3.32 3.69 4.06 Birefringence too close to call a gap of 0.001, within the ±0.003 the engine allows a reading — too close to call 0.008 0.009 0.005 0.009 0.012 Ranges as recorded in gems.json for Sapphire and Tanzanite; every axis is scaled to the pair, so read the tick labels. A gap wider than the instrument's tolerance (RI ±0.010, SG ±0.10, birefringence ±0.003, as the identification engine applies them) means a single clean reading settles it; an overlap, or a gap inside that tolerance, means that instrument cannot, on its own. Hardness is not drawn: a scratch test damages a cut stone. Birefringence is a single recorded value per material.

The Definitive Tests

  1. Refractometer — RI reading. Sapphire reads 1.762-1.770. Tanzanite reads 1.691-1.700. The gap of over 0.06 is wide and unambiguous. A single reading on a standard refractometer separates them.
  2. Dichroscope — pleochroism character. Sapphire shows two colors (dichroic): violet-blue and greenish-blue. Tanzanite shows three colors (trichroic): blue, violet, and burgundy. The burgundy third direction is distinctive and confirms tanzanite. Rotate the stone in different orientations to see all axes.
  3. SG measurement (loose stones). Sapphire SG 3.95-4.03 vs tanzanite 3.35-3.38. Sapphire is substantially heavier. This works well for loose stones using hydrostatic weighing.

Common Mistakes

Frequently Asked Questions

Is tanzanite as durable as sapphire?

No. Tanzanite has a Mohs hardness of 6-7 and perfect cleavage in one direction, making it significantly less durable than sapphire (hardness 9, no cleavage). Tanzanite requires more careful handling, especially in rings.

Can you tell sapphire from tanzanite by color alone?

Experienced gemologists may notice tanzanite's violet-blue shift and stronger pleochroism, but color alone is not reliable for identification. Fine tanzanite can closely resemble fine blue sapphire. Measurement of RI (sapphire 1.762-1.770 vs tanzanite 1.691-1.700) is definitive.

Is all tanzanite heat treated?

Nearly all commercial tanzanite is heat treated to change the brown or brownish-violet rough into the prized blue-violet color. Unheated tanzanite with natural blue color is uncommon and commands a premium from collectors.

Identifying a blue stone? Loupewise walks you through refractometer and dichroscope tests step by step.

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