igneous
Dark volcanic rock, likely basalt
Basalt; fine-grained mafic igneous rock composed mainly of plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectVery dark gray to brown, dull to weakly vitreous luster, irregular fracture, and apparently fine-grained texture; hardness about 6 Mohs and specific gravity roughly 2.8–3.0. The glossy black surface may be a fresh fracture or minor alteration/coating.
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Physical properties
Very dark gray to brown, dull to weakly vitreous luster, irregular fracture, and apparently fine-grained texture; hardness about 6 Mohs and specific gravity roughly 2.8–3.0. The glossy black surface may be a fresh fracture or minor alteration/coating.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows and oceanic crust; specimens may range from recent eruptions to Precambrian formations. Exact identification and age cannot be confirmed from the photograph alone.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, dimension stone, and basalt fiber; attractive dense pieces are collected but usually have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. It commonly contains tiny crystals invisible without magnification, unlike coarse-grained gabbro.
Field identification & locations
Look for a dense, dark, non-layered rock with fine texture and no obvious individual quartz grains; a streak test is typically pale gray. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic crust; a hand lens and magnetism test can help detect iron-rich minerals.
