igneous
Polished black obsidian (likely mahogany or rainbow obsidian)
Volcanic glass; predominantly rhyolitic obsidian, approximately 70–75% SiO₂
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black, vitreous luster, conchoidal fracture, and no visible crystal cleavage. The brown-gold patches and tiny iridescent flashes may reflect iron-oxide inclusions, flow bands, or surface polish rather than crystalline color play.
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Physical properties
Hardness about 5–5.5 Mohs; black, vitreous luster, conchoidal fracture, and no visible crystal cleavage. The brown-gold patches and tiny iridescent flashes may reflect iron-oxide inclusions, flow bands, or surface polish rather than crystalline color play.
Formation & geological history
Rapid cooling of silica-rich lava prevented crystallization, producing volcanic glass, generally Quaternary to recent in age. Common sources include Mexico, the western United States, Armenia, Ethiopia, and Indonesia.
Uses & applications
Used for cabochons, beads, carvings, ornamental objects, and historically sharp cutting tools; not usually used as structural stone. Common polished pieces typically sell for $5–$25, with attractive patterned or rainbow specimens around $30–$100.
Geological facts
Obsidian is technically glass rather than a conventional crystalline mineral. Its exceptionally sharp conchoidal edges were used for prehistoric knives, spear points, and surgical blades.
Field identification & locations
Identify it by its uniformly glassy surface, very sharp curved fracture, black color, and lack of granular crystals; it should not fizz in acid. The specimen appears tumbled or cabochon-polished, so its original fracture surfaces are mostly concealed.
