igneous
Possibly black obsidian or polished basalt
Volcanic glass (obsidian; chiefly SiO₂) or fine-grained basalt
AI-generated identification · may be incorrectSmall, dark gray-black, smooth specimen with a subtly reflective surface; apparent hardness roughly 5–6.5 Mohs. The image is insufficient to confirm luster, crystal structure, cleavage, or density; obsidian is glassy and noncrystalline, while basalt is crystalline and dense.
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Physical properties
Small, dark gray-black, smooth specimen with a subtly reflective surface; apparent hardness roughly 5–6.5 Mohs. The image is insufficient to confirm luster, crystal structure, cleavage, or density; obsidian is glassy and noncrystalline, while basalt is crystalline and dense.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, generally in the Quaternary and younger volcanic record; basalt forms from rapidly cooled mafic lava and is extremely widespread from Precambrian to modern ages.
Uses & applications
Obsidian is used for ornaments, cabochons, tools, and collectibles; basalt is used as aggregate, paving stone, building stone, and mineral wool.
Geological facts
A sharp conchoidal edge strongly supports obsidian; visible tiny crystals, magnetic response, or a dull granular fracture favor basalt or another dark volcanic rock.
Field identification & locations
Common localities include volcanic regions of Mexico, the western United States, Iceland, Japan, and Hawaii. Test cautiously for a glassy conchoidal fracture, streak, magnetism, and specific gravity; exact identification needs a close-up and scale.
