igneous
Dark gray rock fragment, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, commonly with minor olivine and magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, generally dull to weakly vitreous, with interlocking crystals too small to see clearly; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0.
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Physical properties
Fine-grained, dark gray to black, generally dull to weakly vitreous, with interlocking crystals too small to see clearly; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0.
Formation & geological history
Basalt forms when low-silica mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. The small fragment’s precise age and origin cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces may be sold as decorative specimens, but ordinary fragments have negligible commercial value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals. Vesicles, if present, indicate gas bubbles in the lava; this specimen appears compact and may also be a dark metamorphic or sedimentary fragment without testing.
Field identification & locations
Look for a dense, dark, fine-grained texture and occasional tiny feldspar crystals; a streak, magnetism test, hardness test, and thin-section or chemical analysis would improve identification. Common worldwide, especially volcanic regions.
