igneous
Dark gray fine-grained rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, often with minor olivine and magnetite
AI-generated identification · may be incorrectTypically 5–6 Mohs hardness, dark gray to black, dull to weakly glassy luster, very fine-grained interlocking crystals, and no obvious cleavage; density commonly 2.8–3.0 g/cm³. The specimen appears weathered and possibly vesicle-free, though the image is not detailed enough for certainty.
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Physical properties
Typically 5–6 Mohs hardness, dark gray to black, dull to weakly glassy luster, very fine-grained interlocking crystals, and no obvious cleavage; density commonly 2.8–3.0 g/cm³. The specimen appears weathered and possibly vesicle-free, though the image is not detailed enough for certainty.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, volcanic islands, and oceanic crust. Its age can range from geologically recent to billions of years.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, building stone, and basalt fiber; attractive polished pieces may be sold as decorative stone. This small common fragment has minimal monetary value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Related dark rocks such as fine-grained andesite, shale, or slag can look similar.
Field identification & locations
Look for a uniformly dark, heavy, hard surface that does not react with dilute acid; a streak test is usually gray. Common worldwide, especially volcanic regions; a precise identification requires a fresh surface, hand lens, and possibly mineralogical testing.
