igneous
Black obsidian
Volcanic glass; chiefly rhyolitic silica glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectUsually black, with vitreous luster and conchoidal fracture; amorphous rather than crystalline, with Mohs hardness about 5–5.5 and specific gravity around 2.35–2.65. The sharp, glassy edges and lack of visible grains are diagnostic.
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Physical properties
Usually black, with vitreous luster and conchoidal fracture; amorphous rather than crystalline, with Mohs hardness about 5–5.5 and specific gravity around 2.35–2.65. The sharp, glassy edges and lack of visible grains are diagnostic.
Formation & geological history
Forms when silica-rich lava cools extremely rapidly, preventing crystal growth; most specimens are Quaternary to recent, though obsidian occurs in volcanic rocks of many ages. Common at rhyolitic volcanic domes and lava flows.
Uses & applications
Used for ornamental objects, lapidary work, blades, arrowheads, and jewelry; historically valued for exceptionally sharp surgical and cutting edges. It is also collected as a volcanic glass.
Geological facts
The pieces shown appear to be fractured or roughly cut black obsidian, possibly with minor weathering or sheen. Snowflake, mahogany, rainbow, and sheen varieties result from inclusions or optical effects.
Field identification & locations
Commonly found in volcanic regions of Mexico, the western United States, Armenia, Turkey, Iceland, Japan, and New Zealand. Confirm with conchoidal fracture, glassy luster, lack of cleavage, and a streakless or very faint streak; do not confuse with slag or black glass.
