igneous
Polished black obsidian
Volcanic glass, primarily rhyolitic silica (SiO₂)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; uniformly black, vitreous to waxy luster, conchoidal fracture, no visible crystal structure or cleavage. The smooth reflective surface indicates polishing or tumbling.
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Physical properties
Hardness about 5–5.5 Mohs; uniformly black, vitreous to waxy luster, conchoidal fracture, no visible crystal structure or cleavage. The smooth reflective surface indicates polishing or tumbling.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, typically during Quaternary to recent volcanic eruptions. Common sources include Mexico, the western United States, Armenia, and Iceland.
Uses & applications
Used for cabochons, carvings, beads, ornaments, and historically extremely sharp cutting tools; not generally used as structural stone. This appears to be a polished decorative specimen.
Geological facts
Obsidian can produce edges sharper than many steel blades, although they are fragile. Flow textures, inclusions, or a smoky sheen may occur in varieties such as mahogany, snowflake, and rainbow obsidian.
Field identification & locations
Identify by its glassy black appearance, light weight relative to metallic materials, lack of grains, and shell-like fracture; it should scratch glass but not quartz. Confirm with a locality label or hardness test, since black glass and slag can look similar.
