igneous
Tumbled mahogany obsidian
Volcanic glass; rhyolitic obsidian, typically SiO₂-rich with iron-oxide inclusions
AI-generated identification · may be incorrectUsually black and reddish-brown with a glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5. The smooth rounded surface indicates tumbling or polishing; the red-brown color comes from iron oxides or hematite inclusions.
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Physical properties
Usually black and reddish-brown with a glassy luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5. The smooth rounded surface indicates tumbling or polishing; the red-brown color comes from iron oxides or hematite inclusions.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally during Quaternary volcanic activity. Mahogany varieties are especially associated with volcanic regions of the western United States and Mexico.
Uses & applications
Used for cabochons, beads, carvings, worry stones, and decorative objects; historically fashioned into sharp blades and tools. Common polished pieces have modest collector value.
Geological facts
Obsidian is technically a natural volcanic glass rather than a crystalline mineral. Its conchoidal fracture can produce exceptionally sharp edges, sharper than many steel blades at microscopic scale.
Field identification & locations
Identify it by its black-to-mahogany glassy appearance, rounded conchoidal surfaces, lack of grain, and relatively light weight compared with metallic ores. Common sources include Oregon, California, Arizona, Utah, Mexico, Armenia, and Turkey.
