igneous
Polished black stone, likely black obsidian
Obsidian, volcanic glass; primarily silica-rich SiO₂ with minor feldspar, iron, and magnesium
AI-generated identification · may be incorrectBlack, opaque, vitreous luster with a smoothly polished surface; amorphous rather than crystalline, no cleavage, hardness about 5–5.5 Mohs, specific gravity about 2.35–2.60. The image alone cannot exclude black glass, basalt, or dyed stone.
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Physical properties
Black, opaque, vitreous luster with a smoothly polished surface; amorphous rather than crystalline, no cleavage, hardness about 5–5.5 Mohs, specific gravity about 2.35–2.60. The image alone cannot exclude black glass, basalt, or dyed stone.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during volcanic eruptions in the Quaternary and older volcanic settings. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for beads, cabochons, carvings, ornamental objects, and historically exceptionally sharp tools and blades; not generally used as structural stone.
Geological facts
Obsidian often shows conchoidal fracture and may display snowflake, sheen, rainbow, or mahogany varieties. It is natural volcanic glass, not a mineral in the strict crystallographic sense.
Field identification & locations
Identify by glassy luster, uniform dark color, sharp conchoidal edges, and lack of visible grains; a streak, scratch, or magnification test can help distinguish it from basalt and manufactured glass. This appears to be a small tumbled or cut cabochon.
