Amethyst vs Opal
Amethyst is harder (Mohs 7) 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.
Amethyst has the higher refractive index (1.544–1.553 vs 1.450), which a refractometer can use to separate them.
Amethyst is denser (SG 2.65 vs 2.15); a amethyst of the same size weighs about 23% more.
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| Property | Amethyst | Opal |
|---|---|---|
| Species / material | Quartz | Opal (hydrated silica) |
| Mohs hardness | 7 | 5.5–6.5 |
| Refractive index | 1.544–1.553 | 1.450 |
| Specific gravity | 2.65 | 2.15 |
| Crystal system | Trigonal | Amorphous |
| Birefringence | 0.009 | None |
Values are typical ranges from standard gemmological references; individual stones can vary.