igneous
Tumbled black stone, probably black obsidian
Obsidian — volcanic glass, chiefly SiO₂
AI-generated identification · may be incorrectBlack, smooth, and strongly vitreous with a conchoidal-looking surface; hardness about 5–5.5 Mohs, no cleavage, amorphous (noncrystalline) structure, and specific gravity around 2.35–2.ಿಸ; the photograph alone cannot exclude black tourmaline, onyx, or slag glass.
Identified More igneous →
Explore Tumbled black stone, probably black obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Black, smooth, and strongly vitreous with a conchoidal-looking surface; hardness about 5–5.5 Mohs, no cleavage, amorphous (noncrystalline) structure, and specific gravity around 2.35–2.ಿಸ; the photograph alone cannot exclude black tourmaline, onyx, or slag glass.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in young volcanic regions, commonly Quaternary to older Cenozoic deposits.
Uses & applications
Used for ornamental objects, cabochons, beads, tools, and collectible specimens; exceptionally sharp flakes were historically used for cutting tools and surgery prototypes.
Geological facts
Fresh edges can be razor sharp because obsidian fractures conchoidally; snowflake, mahogany, and rainbow varieties result from inclusions or optical effects.
Field identification & locations
Field clues include uniform glassy luster, very sharp curved fracture, lack of visible grains, and inability to scratch glass reliably; common sources include Mexico, the western United States, Armenia, Turkey, and Japan.
