gemstone

Polished opal

Opal, hydrated amorphous silica (SiO₂·nH₂O)

AI-generated identification · may be incorrect

A small polished cabochon showing yellow, orange, and green areas; opal has no crystalline structure and typically has Mohs hardness 5.5–6.5. The photo does not clearly show enough detail to distinguish precious opal from common opal or identify any treatment.

Identified October 13, 2026More gemstone →

Explore Polished opal in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Forms as silica-rich water deposits gel in rock fractures or cavities, then hardens over time; deposits occur in sedimentary and volcanic settings, commonly from the Cretaceous to recent geological periods.

Uses & applications

Used in jewelry and mineral collections; softer opal jewelry is often protected in settings and should be kept away from heat and harsh chemicals.

Geological facts

Precious opal displays shifting spectral color from light diffraction by ordered microscopic silica spheres; common opal lacks this play-of-color.

Field identification & locations

Look for a glassy-to-waxy polish and, in precious opal, color flashes that change with viewing angle. Major sources include Australia, Ethiopia, Mexico, and the United States.