igneous
Polished black gemstone, likely obsidian
Volcanic glass; chiefly silica-rich amorphous SiO₂
AI-generated identification · may be incorrectTypically black, vitreous, and noncrystalline with conchoidal fracture; Mohs hardness about 5–5.5 and specific gravity 2.35–2.70. The blurred image shows a glossy black polished cabochon, but identification is not definitive; it could also be black onyx or another opaque stone.
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Physical properties
Typically black, vitreous, and noncrystalline with conchoidal fracture; Mohs hardness about 5–5.5 and specific gravity 2.35–2.70. The blurred image shows a glossy black polished cabochon, but identification is not definitive; it could also be black onyx or another opaque stone.
Formation & geological history
Obsidian forms when felsic lava cools so rapidly that crystals cannot develop, mainly in Quaternary to older volcanic flows and domes. Common sources include Mexico, the western United States, Armenia, and Iceland.
Uses & applications
Used for jewelry cabochons, carvings, decorative objects, and historically razor-sharp tools and weapons. Common polished pieces generally have modest commercial value.
Geological facts
Its conchoidal edges can be exceptionally sharp; archaeological obsidian artifacts are sometimes traceable to their volcanic source by chemical composition. Snowflake obsidian contains white cristobalite inclusions.
Field identification & locations
Check for conchoidal fracture, uniform glassy luster, and absence of crystal grains; a streak test is usually white or colorless. Black onyx is cryptocrystalline quartz and is harder (Mohs 6.5–7), while glass may show bubbles.
