Opal vs Quartz
| Property | Opal | Quartz |
|---|---|---|
| Formula | SiO2·nH2O | SiO2 |
| Crystal System | — | Trigonal |
| Mineral Class | — | 09 Silicates |
| Mohs Hardness | — | 7.0 |
| Color | — | Colorless, white, purple, pink, brown, black |
| Luster | — | Vitreous |
| Streak | — | White |
| Transparency | — | Transparent |
| Cleavage | — | None |
| Fracture | — | Conchoidal |
Key Differences
- ◆ Quartz is crystalline SiO2; opal is amorphous hydrated SiO2 (SiO2·nH2O) — same composition, different structure.
- ◆ Precious opal displays play-of-color (opalescence); quartz does not.
- ◆ Opal (hardness 5.5–6.5) is softer than quartz (hardness 7).
- ◆ Opal contains up to 20% water; quartz contains no water.
- ◆ Quartz is stable; opal can dehydrate and crack over time.