Topaz vs Opal
Topaz is harder (Mohs 8) than Opal (Mohs 5.5–6.5), so it resists scratching better; Opal is softer than quartz dust (7) and needs more care in everyday rings.
Topaz has the higher refractive index (1.609–1.643 vs 1.450), which a refractometer can use to separate them.
Topaz is denser (SG 3.53 vs 2.15); a topaz of the same size weighs about 64% more.
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| Property | Topaz | Opal |
|---|---|---|
| Species / material | Topaz | Opal (hydrated silica) |
| Mohs hardness | 8 | 5.5–6.5 |
| Refractive index | 1.609–1.643 | 1.450 |
| Specific gravity | 3.53 | 2.15 |
| Crystal system | Orthorhombic | Amorphous |
| Birefringence | 0.008–0.010 | None |
Values are typical ranges from standard gemmological references; individual stones can vary.