igneous
Polished mahogany obsidian (probable)
Volcanic glass; primarily rhyolitic silica glass (SiO₂), with iron-oxide inclusions
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; dark brown to black with mahogany-red and possibly golden patches, vitreous luster, conchoidal fracture, and no cleavage. The rounded shape indicates tumbling or polishing.
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Physical properties
Hardness about 5–5.5 Mohs; dark brown to black with mahogany-red and possibly golden patches, vitreous luster, conchoidal fracture, and no cleavage. The rounded shape indicates tumbling or polishing.
Formation & geological history
Formed when silica-rich lava cooled too rapidly for crystals to grow, producing natural volcanic glass; red-brown areas generally reflect dispersed iron oxides. Most examples are Quaternary to late Cenozoic and commonly originate in volcanic regions of the western United States.
Uses & applications
Used for cabochons, beads, carvings, ornaments, and collectors’ specimens; historically, obsidian was knapped into sharp tools and projectile points.
Geological facts
Mahogany obsidian is a color variety rather than a separate mineral species. Its glassy fracture can produce edges sharper than surgical steel, though this polished piece is not sharp.
Field identification & locations
Identify by its glassy appearance, rounded conchoidal surfaces, lack of visible crystals, and brown-red swirls or patches; distinguish it from jasper by the absence of granular texture. Common sources include Oregon, California, Utah, Mexico, and Armenia.
