Semi-Precious Stones · uncommon
Opal
Opal is famous for its play-of-color, caused by the diffraction of light through microscopic silica spheres. Australia produces 95% of the world's opals.
Also known as: play of color, october birthstone, australian gem.
Opal properties at a glance
| Type | Semi-Precious Stones |
|---|---|
| Chemical formula | SiO2·nH2O |
| Mohs hardness | 5.5 |
| Crystal system | amorphous |
| Luster | vitreous |
| Streak | White |
| Cleavage | None |
| Fracture | Conchoidal |
| Specific gravity | 2.1 |
| Transparency | translucent |
| Common colors | White, Black, Fire, Blue, Pink |
| Rarity | uncommon |
Geology & background
Forms when silica-rich water seeps into rock crevices and evaporates, leaving silica spheres that diffract light.
Ancient Romans considered opal the most precious gem, symbolizing hope and purity.
How to identify Opal
- Check whether the specimen's color and texture match typical Opal examples.
- Photograph a fresh surface in natural light and avoid shadows, glare, or wet surfaces.
- Use hardness, streak, luster, grain size, and locality together; no single trait is enough for every specimen.
Full Opal identification guide: checklist, home tests and look-alikes →
Opal vs similar stones
Not sure it isn't something else? Side-by-side comparisons with the stones most often confused with Opal.
Where Opal is found
Opal is reported from multiple localities or geological settings. Exact locality should be verified from specimen provenance, maps, or field notes.
How much is Opal worth?
| Entry-level specimen | $50 |
|---|---|
| Typical collector specimen | $500 |
| Premium specimen | $10000+ |
Value depends on size, condition, color, crystal quality, locality, rarity, preparation, and provenance. Treat this as educational guidance, not an appraisal.
Common uses
- Jewelry
- Collector specimens
Interesting facts
- Australia produces about 95% of the world's precious opal
- Opals contain up to 20% water
- The Romans called opal 'cupid paederos' (child beautiful as love)
Care & safety
Treat Opal as a collector specimen unless hardness, cleavage, and fracture are confirmed.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a properly rated respirator.
Opal FAQ
What is Opal?
Opal is famous for its play-of-color, caused by the diffraction of light through microscopic silica spheres. Australia produces 95% of the world's opals.
How much is Opal worth?
Entry-level Opal specimens sell for around $50, a typical collector specimen for about $500, and premium pieces for $10000 or more. Value depends on size, condition, color, crystal quality, locality, rarity, preparation, and provenance. Treat this as educational guidance, not an appraisal.
How hard is Opal?
Opal has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
Where is Opal found?
Notable sources of Opal include Australia, Ethiopia, Mexico, Brazil, and USA. Forms when silica-rich water seeps into rock crevices and evaporates, leaving silica spheres that diffract light..
What is Opal used for?
Opal is used for jewelry and collector specimens.
