igneous
Obsidian (likely volcanic glass)
Obsidian; amorphous rhyolitic volcanic glass, typically rich in SiO₂
AI-generated identification · may be incorrectBlack to dark gray, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity about 2.35–2.65. The sharp curved edges and glassy dark surface are characteristic, though the image cannot confirm composition.
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Physical properties
Black to dark gray, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity about 2.35–2.65. The sharp curved edges and glassy dark surface are characteristic, though the image cannot confirm composition.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals do not develop, commonly during Quaternary to older volcanic eruptions. It occurs in volcanic flows, domes, and margins of rhyolitic lava bodies.
Uses & applications
Historically knapped into blades and arrowheads; today used for ornamental objects, jewelry, lapidary work, and occasionally specialized surgical blades. It has limited construction use.
Geological facts
Obsidian can produce exceptionally sharp edges and may show snowflake, banded, mahogany, or sheen varieties. Hydration-rind dating can estimate the age of archaeological artifacts, but requires laboratory analysis.
Field identification & locations
Identify it by its glassy appearance, uniform dark color, and shell-like fracture; it should not show granular crystals. Common sources include Yellowstone, Oregon, California, Mexico, Armenia, and Iceland.
