igneous
Obsidian-like volcanic rock (likely basaltic volcanic glass)
Basaltic obsidian or tachylite; composition chiefly volcanic glass with plagioclase and pyroxene
AI-generated identification · may be incorrectDark brown-black, very fine-grained to glassy, with a dull-to-subvitreous luster and sharp conchoidal-looking fractures. Apparent flow lines and a weathered rusty-brown rind are visible; hardness roughly 5–6, with no obvious cleavage.
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Physical properties
Dark brown-black, very fine-grained to glassy, with a dull-to-subvitreous luster and sharp conchoidal-looking fractures. Apparent flow lines and a weathered rusty-brown rind are visible; hardness roughly 5–6, with no obvious cleavage.
Formation & geological history
An extrusive igneous material formed when iron- and magnesium-rich lava cooled rapidly, locally preserving glass; weathering produced the brown surface. Precise age and locality cannot be determined from the photograph.
Uses & applications
Used historically for sharp cutting tools and occasionally as a decorative or lapidary material; generally low commercial value unless provenance or exceptional texture is established.
Geological facts
The strongly curved, radiating fracture texture is typical of rapidly cooled volcanic glass or glass-rich lava, though a dense, fine-grained basalt is also possible.
Field identification & locations
Check whether fresh edges are glass-sharp, whether the surface scratches glass, and whether a streak is absent or gray-black. Common examples occur in volcanic regions such as Mexico, the western United States, Iceland, and Turkey; laboratory testing is needed to distinguish tachylite from basalt or industrial slag.
