igneous
Obsidian (likely)
Volcanic glass; primarily silica-rich rhyolitic composition (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectUsually black, with vitreous to dull luster and conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, amorphous rather than crystalline, and specific gravity about 2.3–2.6. The dark, glassy fractured appearance is consistent with obsidian, though lighting limits certainty.
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Physical properties
Usually black, with vitreous to dull luster and conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, amorphous rather than crystalline, and specific gravity about 2.3–2.6. The dark, glassy fractured appearance is consistent with obsidian, though lighting limits certainty.
Formation & geological history
Forms when felsic lava cools so rapidly that crystals cannot grow, generally during Quaternary to older volcanic eruptions. Commonly associated with rhyolite domes and lava flows.
Uses & applications
Used historically for extremely sharp blades and arrowheads; today it is collected, carved, used in jewelry, and occasionally made into surgical blades or decorative objects.
Geological facts
Fresh fractures can produce edges sharper than steel blades, but obsidian is brittle and readily chips. Snowflake obsidian contains white cristobalite inclusions, not visible here.
Field identification & locations
Identify by its glassy black color, curved shell-like fracture, sharp edges, and lack of visible grains; test cautiously because it can cut skin. Common localities include Iceland, Mexico, the western United States, Armenia, and Turkey.
