igneous
Obsidian (likely black obsidian)
Volcanic glass; predominantly rhyolitic silica-rich melt (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectTypically black, dark brown, or greenish-black with vitreous luster and conchoidal fracture; amorphous rather than crystalline, with Mohs hardness about 5–5.5 and specific gravity 2.35–2.65. The glossy, sharply curved fracture surfaces strongly fit obsidian, though the image alone cannot exclude industrial glass or tek…
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Physical properties
Typically black, dark brown, or greenish-black with vitreous luster and conchoidal fracture; amorphous rather than crystalline, with Mohs hardness about 5–5.5 and specific gravity 2.35–2.65. The glossy, sharply curved fracture surfaces strongly fit obsidian, though the image alone cannot exclude industrial glass or tektite.
Formation & geological history
Forms when silica-rich lava cools so quickly that crystals cannot grow, mainly during volcanic eruptions in the Cenozoic and older periods. Common localities include Iceland, Mexico, the western United States, Armenia, Ethiopia, and Japan.
Uses & applications
Used historically for exceptionally sharp cutting tools and arrowheads; today it is collected, carved, and occasionally fashioned into cabochons or jewelry. Large masses have limited construction value.
Geological facts
Fresh edges can be sharper than surgical steel because fracture occurs at a microscopic scale. Snowflake, sheen, rainbow, and mahogany varieties reflect inclusions or flow textures.
Field identification & locations
Identify by its glassy luster, lack of visible grains, conchoidal fracture, and razor-sharp edges; a streak test is usually inconclusive. Collector value is commonly $1–$10 for a small ordinary fragment, or $10–$100+ for attractive patterned or polished material.
