Properties
| Category | Mineraloid |
|---|---|
| Formula | Hydrated silica. SiO2·nH2O |
| Crystal system | Amorphous |
| Color | Colorless, white, yellow, red, orange, green, brown, black, blue, pink |
| Crystal habit | Irregular veins, in masses, in nodules |
| Cleavage | None |
| Fracture | Conchoidal to uneven |
| Mohs scale hardness | 5.5–6 |
| Luster | Subvitreous to waxy |
| Streak | White |
| Diaphaneity | opaque, translucent, transparent |
| Specific gravity | 2.15+0.08; −0.90 |
| Density | 2.09 g/cm3 |
From the Wikipedia article
Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.
Source
Source record: crystal-compendium/opal/minerals.json. Fetched from English Wikipedia by the operator 2 October 2026.
https://en.wikipedia.org/wiki/Opal — revision id 1366758276. Text and properties from Wikipedia, licensed CC BY-SA 4.0.