gemstone

Polished boulder opal

Opal, hydrated amorphous silica (SiO₂·nH₂O)

AI-generated identification · may be incorrect

Yellow-brown polished specimen with flowing dark brown/gray banding and possible subdued play-of-color; hardness 5.5–6.5 Mohs, waxy to vitreous luster, no cleavage, and amorphous (noncrystalline) structure. The pattern may also indicate a boulder-opal matrix rather than pure opal.

Identified August 28, 2026More gemstone →

Explore Polished boulder 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 when silica-rich groundwater fills cavities and fractures, commonly in weathered volcanic rocks; Australian opal deposits are mainly Cretaceous, about 65–145 million years old. Iron-stained host rock produces the brown matrix and bands.

Uses & applications

Used in cabochon jewelry, ornamental carvings, and mineral collections; porous material may be stabilized or treated. Value depends strongly on play-of-color, pattern, durability, and whether it is natural or treated.

Geological facts

Opal contains significant water and can craze from heat or sudden drying. Boulder opal retains ironstone or other host rock, often creating attractive natural contrast.

Field identification & locations

Identify by its low hardness, waxy appearance, curved fracture, lightweight feel, and—if present—spectral flashes when rotated; a close photo cannot confirm play-of-color or treatment. Common sources include Queensland, Australia, Ethiopia, Mexico, and Nevada.