Opal
| Colour | White, Clear, Yellow, Red |
|---|---|
| Mohs hardness | 5.5–6 |
| Lustre | Subvitreous to waxy |
| Streak | White |
| Crystal system | Amorphous |
| Transparency | opaque, translucent, transparent |
| Cleavage | None |
| Fracture | Conchoidal to uneven |
| Chemical formula | Hydrated silica. SiO 2 ·nH 2 O |
| Specific gravity | 2.15 |
What is Opal?
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.
How to identify Opal
- Lustre: Subvitreous to waxy.
- Hardness: Mohs 5.5–6 — about as hard as a steel knife.
- Streak: White.
- Habit: Amorphous crystal system.
Opal in different forms
Stones often confused with Opal
Frequently asked questions
How hard is Opal?
Opal is Mohs 5.5–6 on the hardness scale.
What colour is Opal?
Opal is typically white, clear, yellow, red (Colorless, white, yellow, red, orange, green, brown, black, blue, pink).