Opal
Opal is a gemstone with the chemical formula SiO2·nH2O. It has a Mohs hardness of 5.5 to 6.5 and vitreous to waxy luster.
Opal properties
| Type | Gemstone |
|---|---|
| Chemical formula | SiO2·nH2O |
| Mohs hardness | 5.5 to 6.5 |
| Luster | Vitreous to waxy |
| Typical colors | Colorless, white, black, and various combinations with hues of red, orange, yellow, green, blue, and violet |
| Formation | Formed from the silica gel that fills cavities or cracks in sedimentary rocks |
| Rarity | 7 / 10 |
Mohs hardness 5.5–6.5 — fairly hard
1 · Talc5 · Apatite10 · Diamond
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