igneous
Polished black stone, likely black obsidian
Obsidian (volcanic glass), primarily rhyolitic silica glass
AI-generated identification · may be incorrectTypically black, glossy vitreous luster, no visible crystals or cleavage, conchoidal fracture, and Mohs hardness about 5–5.5; the rounded shape indicates tumbling or polishing. The image alone cannot exclude black agate, onyx, or tourmaline.
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Physical properties
Typically black, glossy vitreous luster, no visible crystals or cleavage, conchoidal fracture, and Mohs hardness about 5–5.5; the rounded shape indicates tumbling or polishing. The image alone cannot exclude black agate, onyx, or tourmaline.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, mainly during Quaternary to recent volcanic eruptions. Common sources include Armenia, Mexico, Iceland, the western United States, and Japan.
Uses & applications
Used for ornaments, cabochons, beads, carvings, and historically extremely sharp tools and blades; it has limited construction use compared with crystalline volcanic rocks.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks a crystalline structure. Snowflake obsidian contains devitrified cristobalite, but no pattern is evident here.
Field identification & locations
A uniformly glassy surface, sharp conchoidal chips, and absence of crystal grains support obsidian; a streak test is usually unhelpful. Confirm with a loupe and hardness testing, avoiding damage to the polished piece.
