mineral
Polished dendritic opal or common opal
Opal, hydrated amorphous silica (SiO₂·nH₂O), likely dendritic common opal
AI-generated identification · may be incorrectWhite to translucent with black branching or blotchy inclusions, vitreous-to-waxy luster, no true crystal structure, conchoidal fracture, and Mohs hardness about 5.5–6.5. The black material may be manganese-oxide dendrites or dark mineral inclusions.
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Physical properties
White to translucent with black branching or blotchy inclusions, vitreous-to-waxy luster, no true crystal structure, conchoidal fracture, and Mohs hardness about 5.5–6.5. The black material may be manganese-oxide dendrites or dark mineral inclusions.
Formation & geological history
Forms as silica-rich groundwater fills cavities and fractures, commonly in volcanic rocks; dendritic patterns develop from manganese or iron oxides. Opal is geologically young compared with most crystalline minerals, often forming from the Cenozoic onward.
Uses & applications
Used as a polished ornamental stone, cabochon, and collectible; non-gem common opal has limited industrial use. It is generally less valuable than precious opal because it lacks play-of-color.
Geological facts
The specimen appears tumbled or cabochon-polished; its black markings are likely inclusions rather than natural opal color. Similar-looking material may also be white jasper with black manganese staining.
Field identification & locations
Identify by its low hardness, waxy appearance, irregular black dendrites, and lack of iridescent play-of-color; avoid prolonged heat and sudden temperature changes. Common sources include Australia, Mexico, Ethiopia, Peru, and the western United States.
