Sapphire vs Tanzanite: How to Tell Them Apart
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 |
The Definitive Tests
- 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.
- 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.
- 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
- Confusing dichroism with trichroism. Both sapphire and tanzanite are pleochroic, so simply confirming pleochroism is not sufficient. Count the number of distinct colors: sapphire shows two (dichroic), tanzanite shows three (trichroic). The burgundy/red-brown third axis is unique to tanzanite.
- Not accounting for heat treatment. Nearly all commercial tanzanite is heat treated to remove brownish body color. This is standard and expected. It does not affect identification — the RI, SG, and pleochroism remain the same after treatment.
- Assuming durability is similar. Sapphire (hardness 9) is suitable for daily-wear rings. Tanzanite (hardness 6-7, with perfect cleavage) is vulnerable to knocks and thermal shock. This matters for jewelry setting and care advice, not identification — but it is a common point of confusion for consumers.
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? GemID walks you through refractometer and dichroscope tests step by step.
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