igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass; predominantly SiO₂)
AI-generated identification · may be incorrectTypically black, very glossy, and noncrystalline with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The smooth, reflective appearance may also fit polished basalt or shungite, so identification from the image alone is limited.
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Physical properties
Typically black, very glossy, and noncrystalline with conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The smooth, reflective appearance may also fit polished basalt or shungite, so identification from the image alone is limited.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in volcanic settings younger than a few million years. Common sources include Mexico, the western United States, Iceland, Armenia, and Japan.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, tools, and historically exceptionally sharp blades. Polished pieces are inexpensive unless unusually patterned, translucent, or provenance-rich.
Geological facts
Obsidian is technically a natural volcanic glass rather than a crystalline mineral. Its conchoidal fracture can produce edges sharper than surgical steel, though they are brittle.
Field identification & locations
Look for uniform glassy luster, curved fracture surfaces, and absence of visible grains; a steel knife usually does not scratch it readily. A strong magnetic response or a streak would suggest another material, while bubbles may indicate manufactured glass.
