igneous
Black tumbled stone, likely obsidian
Volcanic glass (obsidian), primarily SiO₂
AI-generated identification · may be incorrectTypically black, vitreous/glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; the rounded shape indicates tumbling or polishing. The photograph alone cannot distinguish obsidian confidently from black tourmaline, onyx, or glass.
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Physical properties
Typically black, vitreous/glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; the rounded shape indicates tumbling or polishing. The photograph alone cannot distinguish obsidian confidently from black tourmaline, onyx, or glass.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystallization, generally in geologically young volcanic fields. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for ornamental carvings, cabochons, beads, and jewelry; historically fashioned into exceptionally sharp blades and arrowheads. Common polished pieces have modest collecting value.
Geological facts
Its sharp conchoidal edges can be sharper than surgical steel, although it is brittle. Snowflake, rainbow, sheen, and mahogany varieties contain distinctive inclusions or color patterns.
Field identification & locations
Check for curved shell-like fracture, glassy luster, uniform density, and absence of bubbles or mold seams; a hardness or streak test can help, but avoid damaging a polished specimen. Black obsidian is common in volcanic regions worldwide.
