igneous
Dark mafic rock, likely basalt
Basalt; primarily plagioclase feldspar, clinopyroxene, and possibly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to weakly granular luster; typically ~5–6 Mohs, with interlocking microscopic crystals and no obvious cleavage. The rounded, irregular surface may indicate weathering, abrasion, or a tumbled specimen.
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Physical properties
Fine-grained, dark gray to black, dull to weakly granular luster; typically ~5–6 Mohs, with interlocking microscopic crystals and no obvious cleavage. The rounded, irregular surface may indicate weathering, abrasion, or a tumbled specimen.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. It ranges from ancient Archean rocks to modern eruptions; age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, concrete additive, and dimension stone; attractive fine-grained pieces are collected but have limited gem value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later be filled by minerals such as quartz, calcite, or zeolites.
Field identification & locations
Identify it by its heavy, dark, fine-grained appearance, lack of visible large crystals, and inability to scratch it easily with a steel knife. Common worldwide, especially near volcanic regions and oceanic crust; definitive identification may require a fresh surface or thin-section analysis.
