igneous
Polished mahogany obsidian
Volcanic glass; rhyolitic obsidian, commonly with iron-oxide coloration
AI-generated identification · may be incorrectBlack to reddish-brown, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.60. The smooth surface indicates polishing or tumbling.
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Physical properties
Black to reddish-brown, vitreous luster, conchoidal fracture, no visible crystals or cleavage; Mohs hardness about 5–5.5 and specific gravity about 2.35–2.60. The smooth surface indicates polishing or tumbling.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally during Quaternary-to-recent volcanic eruptions. Brown-red areas result from iron oxides or microscopic inclusions rather than separate crystals.
Uses & applications
Used for cabochons, carvings, beads, ornaments, and collectible polished specimens; historically, sharp obsidian flakes served as cutting tools.
Geological facts
Obsidian is technically a natural glass rather than a true crystalline mineral. Mahogany obsidian is named for its reddish-brown, mahogany-like patches against black glass.
Field identification & locations
Identify by its glassy appearance, smooth curved fracture surfaces, lack of grains, and sharp edges when freshly broken; common sources include Mexico, the western United States, Armenia, and Japan. A streak test is usually inconclusive because the specimen is polished.
