igneous
Polished black stone, likely obsidian
Volcanic glass; primarily silica-rich, noncrystalline SiO₂
AI-generated identification · may be incorrectTypically black, vitreous to glassy luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, specific gravity 2.35–2.65. The rounded, polished form prevents confident distinction from black agate, basalt, or manufactured glass.
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Physical properties
Typically black, vitreous to glassy luster, conchoidal fracture, and no visible crystals; Mohs hardness about 5–5.5, specific gravity 2.35–2.65. The rounded, polished form prevents confident distinction from black agate, basalt, or manufactured glass.
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, and Armenia.
Uses & applications
Used for ornamental objects, cabochons, beads, tools, and historically razor-sharp blades; not generally used as structural stone.
Geological facts
Fresh obsidian can produce extremely sharp conchoidal edges, sharper than many steel blades. Brown or golden flow bands may occur, while snowflake obsidian contains cristobalite inclusions.
Field identification & locations
Look for natural flow banding, conchoidal fracture, glassy luster, and sharp edges; streak and transparency tests are less useful on a polished specimen. A gemological test or refractive-index measurement is needed to separate it from glass or black agate.
