igneous
Obsidian (likely mahogany or black obsidian)
Volcanic glass; chiefly silica-rich rhyolitic composition, approximately SiO₂-rich melt
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black to brown with glossy vitreous luster and conchoidal fracture. It is noncrystalline or cryptocrystalline, lacks cleavage, and typically has specific gravity around 2.35–2.65.
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Physical properties
Hardness about 5–5.5 Mohs; black to brown with glossy vitreous luster and conchoidal fracture. It is noncrystalline or cryptocrystalline, lacks cleavage, and typically has specific gravity around 2.35–2.65.
Formation & geological history
Forms when felsic lava cools so rapidly that crystals cannot grow, commonly during Quaternary to recent volcanic eruptions. Brown-red patches suggest iron-oxide-rich mahogany obsidian.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp blades and tools; not generally used as structural stone.
Geological facts
Its curved, shell-like fractures can produce edges sharper than surgical steel, though they are brittle. It may resemble dark glass but natural surfaces commonly show flow banding or inclusions.
Field identification & locations
Identify by its glassy luster, very sharp conchoidal edges, lack of visible grains, and inability to scratch glass consistently; common sources include Mexico, the western United States, Armenia, Turkey, and Iceland.
