igneous
Polished black stone, likely obsidian
Volcanic glass (obsidian), primarily silica-rich amorphous SiO₂
AI-generated identification · may be incorrectBlack, glossy vitreous luster with smooth polished surfaces; hardness about 5–5.5 Mohs, no cleavage, conchoidal fracture, and specific gravity around 2.3–2.6. The image is slightly blurred, so black tourmaline or polished basalt are alternative possibilities.
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Physical properties
Black, glossy vitreous luster with smooth polished surfaces; hardness about 5–5.5 Mohs, no cleavage, conchoidal fracture, and specific gravity around 2.3–2.6. The image is slightly blurred, so black tourmaline or polished basalt are alternative possibilities.
Formation & geological history
Obsidian forms when felsic lava cools so rapidly that crystals cannot grow, generally during relatively young volcanic eruptions from the Cenozoic to present. Common sources include Mexico, the western United States, Iceland, Armenia, and Japan.
Uses & applications
Used for ornaments, cabochons, beads, carvings, and decorative objects; sharp natural edges were historically used for cutting tools. Ordinary polished pieces usually have modest value.
Geological facts
Obsidian is technically natural volcanic glass rather than a crystalline mineral. Snowflake, rainbow, sheen, and mahogany varieties display distinctive inclusions or optical effects.
Field identification & locations
Test for a glassy luster, conchoidal edges, and lack of visible crystals; it should not scratch easily with a steel nail. Common market pieces are sold as polished black obsidian, though confirmation requires clearer imaging or testing.
