igneous
Polished black stone, likely black obsidian
Volcanic glass; predominantly rhyolitic silica glass (SiO₂-rich)
AI-generated identification · may be incorrectVery dark green-black, smooth and highly vitreous, with no visible crystals; hardness about 5–5.5 Mohs, conchoidal fracture, and specific gravity around 2.3–2.6. The polished, shaped form appears tumbled or cabochon-cut.
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Physical properties
Very dark green-black, smooth and highly vitreous, with no visible crystals; hardness about 5–5.5 Mohs, conchoidal fracture, and specific gravity around 2.3–2.6. The polished, shaped form appears tumbled or cabochon-cut.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in volcanic settings during the Quaternary and older Cenozoic. The specimen has been human-polished and shaped.
Uses & applications
Used for carvings, cabochons, beads, ornamental objects, and historically sharp tools and blades. Common polished pieces have modest decorative value.
Geological facts
Obsidian can produce exceptionally sharp fracture edges and may show sheen, snowflake patterns, or translucent bands. This example could also be black glassy basalt or manufactured glass, which cannot be confirmed from the image alone.
Field identification & locations
Look for natural conchoidal fracture, glassy luster, uniform density, and absence of bubbles; bubbles or perfectly homogeneous appearance may indicate man-made glass. Common sources include Mexico, the western United States, Armenia, Italy, and Japan.
