Opal
| Colour | White, Clear, Yellow, Red, Orange, Green, Brown, Black, Blue, Pink |
|---|---|
| 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 value
Opal is a fine gemstone priced per carat by quality — colour, clarity, cut, carat and origin — not by weight in grams. Typical market ranges:
| Quality | Price per carat |
|---|---|
| Commercial | $10–$300 / ct |
| Good | $50–$2,000 / ct |
| Fine | $500–$8,000 / ct |
| Top | $3,500–$30,000 / ct |
Ranges cross-validated across two independent research passes against gemstone price guides, 2026-06 — sources: gia.edu, opalauctions.com. Colour saturation, clarity, cut and origin move the price enormously, and treatments or lab-grown stones sell for a fraction — so this is a guide, not an appraisal. Have anything valuable checked by a certified gemmologist. See the full crystal & gemstone value guide.
