Spinel vs Opal
Spinel 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.
Spinel has the higher refractive index (1.718 vs 1.450), which a refractometer can use to separate them.
Spinel is denser (SG 3.6 vs 2.15); a spinel of the same size weighs about 67% more.
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| Property | Spinel | Opal |
|---|---|---|
| Species / material | Spinel | Opal (hydrated silica) |
| Mohs hardness | 8 | 5.5–6.5 |
| Refractive index | 1.718 | 1.450 |
| Specific gravity | 3.6 | 2.15 |
| Crystal system | Cubic | Amorphous |
| Birefringence | None (isotropic) | None |
Values are typical ranges from standard gemmological references; individual stones can vary.