igneous
Polished black stone, likely obsidian
Volcanic glass (approximately rhyolitic composition; mostly SiO₂)
AI-generated identification · may be incorrectBlack, vitreous luster, apparently polished and rounded; hardness about 5–5.5 Mohs, conchoidal fracture, no visible crystals or cleavage, specific gravity about 2.35–2.6.
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Physical properties
Black, vitreous luster, apparently polished and rounded; hardness about 5–5.5 Mohs, conchoidal fracture, no visible crystals or cleavage, specific gravity about 2.35–2.6.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, generally during Quaternary-to-Holocene volcanic eruptions. The specimen’s polished surface obscures diagnostic natural fracture features.
Uses & applications
Used for ornamental objects, cabochons, beads, tools, and collectibles; historically shaped into extremely sharp cutting implements.
Geological facts
Obsidian commonly shows flow banding, snowflake patterns, or sheen varieties, but none is clearly visible here. Similar-looking black glass, basalt, or slag cannot be excluded from the photograph alone.
Field identification & locations
Likely obsidian if it has a glassy luster and sharp conchoidal chips; test cautiously for uniform glassiness and absence of bubbles or metallic grains. Common sources include Mexico, the western United States, Iceland, Turkey, and Armenia.
