igneous
Obsidian-like volcanic glass (likely slag or natural obsidian)
Obsidian, primarily amorphous volcanic silica glass (approximately SiO₂); identification is uncertain from an image
AI-generated identification · may be incorrectBlack to brown, glassy-to-dull luster with sharp conchoidal-looking fractures and possible flow banding. Obsidian has Mohs hardness about 5–5.5, no cleavage, and specific gravity near 2.35; the rough brown coating may be weathering or iron staining.
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Physical properties
Black to brown, glassy-to-dull luster with sharp conchoidal-looking fractures and possible flow banding. Obsidian has Mohs hardness about 5–5.5, no cleavage, and specific gravity near 2.35; the rough brown coating may be weathering or iron staining.
Formation & geological history
Natural obsidian forms when silica-rich lava cools rapidly, commonly in Pleistocene to recent volcanic flows. The visibly frothy or irregular texture could also indicate industrial furnace slag, which is not a natural rock.
Uses & applications
Obsidian is used for ornamental objects, tools, and occasionally jewelry; historically it was knapped into exceptionally sharp blades. Common pieces generally have modest commercial value.
Geological facts
A fresh fracture can be razor sharp and strongly curved (conchoidal). Vesicles, metallic droplets, or abundant bubbles would favor slag rather than obsidian.
Field identification & locations
Check for uniform glass, curved fracture surfaces, and flow bands; test hardness against a steel knife and inspect for bubbles or metal. Natural obsidian occurs near rhyolitic volcanic centers such as the western United States, Mexico, Iceland, and Japan.
