mineral

Common opal (likely dendritic or moss opal)

Hydrated amorphous silica, SiO₂·nH₂O

AI-generated identification · may be incorrect

White to gray, translucent chalcedony-like material with dark green-black inclusions and rusty iron staining; waxy to dull luster, no cleavage, conchoidal fracture, and Mohs hardness about 5.5–6.5. The specimen appears roughly tumbled or naturally rounded.

Identified August 19, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater deposits hydrated silica in cavities, fractures, or volcanic host rock; inclusions may be manganese or iron oxides and dendritic minerals. Opal is geologically young, commonly forming from the Cenozoic to the present.

Uses & applications

Used as a lapidary stone, cabochon, ornamental material, and collector specimen; common-quality pieces have limited jewelry value because opal can dehydrate, craze, and scratch relatively easily.

Geological facts

The dark branching or mossy patterns are inclusions rather than true crystal growth, since opal lacks a crystalline lattice. It may also be described commercially as dendritic opal or moss opal, though visual identification alone cannot confirm the exact variety.

Field identification & locations

Look for low density, waxy appearance, conchoidal fracture, and absence of cleavage; it should not fizz in dilute acid. Common sources include Mexico, Australia, Ethiopia, Peru, and the western United States.