igneous
Black obsidian
Volcanic glass, primarily rhyolitic silica glass (SiO₂-rich, noncrystalline)
AI-generated identification · may be incorrectTypically jet black, vitreous to glassy luster, conchoidal fracture, and no cleavage or visible crystals; Mohs hardness about 5–5.5, specific gravity roughly 2.35–2.65. The specimen appears to be a naturally fractured or roughly shaped dark glassy fragment.
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Physical properties
Typically jet black, vitreous to glassy luster, conchoidal fracture, and no cleavage or visible crystals; Mohs hardness about 5–5.5, specific gravity roughly 2.35–2.65. The specimen appears to be a naturally fractured or roughly shaped dark glassy fragment.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary volcanic eruptions, though older deposits also occur. Common sources include volcanic regions of Mexico, the western United States, Iceland, Armenia, and the Mediterranean.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp blades and arrowheads; it has limited construction use. Valuable specimens may show sheen, banding, or unusual colors such as mahogany or snowflake patterns.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks a crystal lattice. Fresh fracture edges can be sharper than high-quality steel surgical edges, although they are brittle.
Field identification & locations
Identify by its uniform black color, glassy luster, curved conchoidal fractures, and lack of grains; it may resemble slag or dark glass, so check for natural flow texture and bubbles. Common material is found near rhyolitic lava flows.
