igneous
Polished black stone, possibly obsidian
Volcanic glass (obsidian), chiefly amorphous SiO₂
AI-generated identification · may be incorrectThe glossy black, smoothly polished specimen resembles obsidian; typical hardness is 5–5.5 Mohs, vitreous luster, conchoidal fracture, and no cleavage. Its exact identity cannot be confirmed from the photograph; black agate, basalt, or industrial glass are alternatives.
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Physical properties
The glossy black, smoothly polished specimen resembles obsidian; typical hardness is 5–5.5 Mohs, vitreous luster, conchoidal fracture, and no cleavage. Its exact identity cannot be confirmed from the photograph; black agate, basalt, or industrial glass are alternatives.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot develop, mainly in volcanic regions from the Quaternary and older Cenozoic eras. It commonly occurs as lava flows, domes, and volcanic tuffs.
Uses & applications
Used for ornaments, cabochons, beads, sculptures, and historically extremely sharp blades and arrowheads. It has limited construction use compared with crystalline volcanic rocks.
Geological facts
Obsidian is a natural glass rather than a conventionally crystalline mineral. Thin edges can be exceptionally sharp, and some varieties display snowflake, sheen, or rainbow effects.
Field identification & locations
Look for uniform glassy luster, conchoidal fractures, curved sharp edges, and lack of visible grains; a streak test is usually inconclusive. Common sources include Mexico, the western United States, Armenia, Turkey, and Japan.
