igneous
Tumbled black obsidian (likely)
Volcanic glass; chiefly silica-rich rhyolitic glass (SiO₂)
AI-generated identification · may be incorrectTypically black to deep brown with vitreous luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; specific gravity approximately 2.35–2.65. The glossy, smoothly shaped pieces appear tumbled or polished.
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Physical properties
Typically black to deep brown with vitreous luster, conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; specific gravity approximately 2.35–2.65. The glossy, smoothly shaped pieces appear tumbled or polished.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Quaternary volcanic eruptions. It occurs in volcanic flows, domes, and lava margins, especially in the western United States, Mexico, Iceland, Japan, and the Mediterranean.
Uses & applications
Used for ornaments, cabochons, carvings, jewelry, and collectibles; historically fashioned into exceptionally sharp cutting tools and arrowheads. 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 an ordered crystal structure. Flow bands, translucent edges, snowflake patterns, or golden/sheen effects may increase desirability.
Field identification & locations
Identify it by its uniform glassy appearance, sharp conchoidal edges, and absence of grains or cleavage; unlike man-made glass, natural pieces may show flow banding or tiny inclusions. The image could also represent polished black glass or another dark stone, so confirmation requires examination or testing.
