igneous
Polished black gemstone, likely black obsidian
Obsidian (volcanic glass), chiefly silica-rich amorphous SiO₂
AI-generated identification · may be incorrectTypically glassy, opaque black with strong vitreous luster; hardness about 5–5.5 Mohs, no crystal structure or cleavage, and specific gravity around 2.35–2.65. The rounded, highly polished form prevents definitive identification from the image; black tourmaline, onyx, or dyed agate are alternatives.
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Physical properties
Typically glassy, opaque black with strong vitreous luster; hardness about 5–5.5 Mohs, no crystal structure or cleavage, and specific gravity around 2.35–2.65. The rounded, highly polished form prevents definitive identification from the image; black tourmaline, onyx, or dyed agate are alternatives.
Formation & geological history
Forms when felsic volcanic lava cools so rapidly that crystals cannot grow, generally during Cenozoic to recent volcanic activity. Common sources include Mexico, the western United States, Armenia, Turkey, and Japan.
Uses & applications
Used for cabochons, beads, carvings, ornaments, and historically exceptionally sharp cutting tools; it has limited construction use. Common polished pieces generally sell for about $5–$25, with larger or unusually patterned specimens worth more.
Geological facts
Obsidian may show snowflake, mahogany, rainbow, or sheen varieties caused by inclusions or optical effects. Fresh fractures are characteristically curved and razor-sharp (conchoidal).
Field identification & locations
A glassy surface, uniform black color, and conchoidal fracture support obsidian; it feels lighter than many metallic-looking stones and will not show crystal faces. A scratch test, streak test, or magnification is needed to distinguish it confidently from black minerals or glass.
