igneous
Black obsidian (likely polished)
Volcanic glass; chiefly rhyolitic silica-rich glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectGlossy black, glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; typically dense, with no cleavage. The image suggests a polished or tumbled piece, though exact identification is limited by its surface.
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Physical properties
Glossy black, glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; typically dense, with no cleavage. The image suggests a polished or tumbled piece, though exact identification is limited by its surface.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly at volcanic lava flows, domes, and margins. Most specimens are Quaternary to relatively young in geological age.
Uses & applications
Used for ornamental objects, cabochons, beads, polished spheres, and historically sharp cutting tools; less commonly used as a decorative facing stone.
Geological facts
Obsidian can produce exceptionally sharp conchoidal edges and may show sheen, snowflake, mahogany, or rainbow varieties. It is technically a volcanic glass rather than a fully crystalline rock.
Field identification & locations
Identify by its uniformly glassy appearance, sharp curved fracture, lack of grains, and ability to scratch glass; black tourmaline and onyx are possible look-alikes. Common sources include Mexico, the western United States, Armenia, Turkey, and Japan.
