igneous
Black volcanic rock, possibly basalt or obsidian
Likely basalt (fine-grained mafic silicate rock); obsidian is a possible alternative
AI-generated identification · may be incorrectDark black, irregular angular specimen with a dull-to-subvitreous luster; apparent fine grain and fracture surfaces are insufficiently clear for confident identification. Basalt hardness ~6 Mohs, density ~2.8–3.0 g/cm³; obsidian is ~5–5.5 Mohs and commonly has glassy conchoidal fracture.
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Physical properties
Dark black, irregular angular specimen with a dull-to-subvitreous luster; apparent fine grain and fracture surfaces are insufficiently clear for confident identification. Basalt hardness ~6 Mohs, density ~2.8–3.0 g/cm³; obsidian is ~5–5.5 Mohs and commonly has glassy conchoidal fracture.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near the surface, commonly in volcanic flows; obsidian forms when silica-rich lava quenches as volcanic glass. No geological age can be determined from the photograph.
Uses & applications
Basalt is used as crushed aggregate, paving stone, and construction material; quality obsidian is used for carvings, tools, and occasional jewelry.
Geological facts
Basalt usually shows tiny crystals under magnification, whereas obsidian is uniformly glassy and breaks with sharp curved edges; a streak or scratch test can help distinguish them.
Field identification & locations
Common worldwide, especially in volcanic regions. Inspect a fresh surface with a loupe: visible grains favor basalt, while glassy translucence at thin edges and conchoidal fractures favor obsidian; avoid relying on color alone.
