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Obsidian vs Basalt: Glass, Crystals and Fracture

Obsidian and basalt are volcanic rocks that can both be black, dense-looking, vesicular, and sharply fractured. Obsidian is volcanic glass, usually of rhyolitic composition, produced when viscous lava cools without substantial crystal growth. Basalt is a mafic volcanic rock that is normally fine crystalline and commonly contains plagioclase, pyroxene, or olivine. Basaltic glass exists, especially at rapidly chilled margins, so texture and context should be combined. A uniformly glassy surface with strong conchoidal fracture and thin translucent edges supports obsidian. A duller fine-grained groundmass with tiny crystals, olivine, plagioclase, or abundant weathered vesicles supports basalt. Do not use black colour alone. Use an already broken surface and avoid making a fresh edge; both rocks can be sharp. Inspect the groundmass with a hand lens. Uniform glass supports obsidian, while a felted or speckled microcrystalline texture supports basalt. Look for smooth curved conchoidal surfaces and thin translucent edges without touching sharp margins. Search for olivine, plagioclase, pyroxene, filled vesicles, or a fine crystalline matrix, which favour basalt. Record whether the specimen comes from a glassy silicic flow, a basaltic flow interior, or a rapidly chilled flow margin. Obsidian edges can be sharper than a steel blade. Do not pressure-flake, strike, or run a finger along a fresh edge. Basaltic glass is uncommon but real, so a glassy texture does not by itself prove rhyolitic obsidian. Industrial glass and slag can resemble volcanic glass. Bubbles, inclusions, site context, and laboratory chemistry may be needed to separate manufactured material.