igneous
Likely basalt fragment
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to mildly vitreous, with no visible crystals or cleavage; typically Mohs 6, specific gravity about 2.8–3.0. The straight sides suggest it may be cut or shaped rather than naturally broken.
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Physical properties
Dark gray to black, fine-grained, dull to mildly vitreous, with no visible crystals or cleavage; typically Mohs 6, specific gravity about 2.8–3.0. The straight sides suggest it may be cut or shaped rather than naturally broken.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Most examples range from Precambrian to modern, depending on locality.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railway ballast, paving, and building stone; attractive polished pieces may be sold as decorative material. This small fragment has minimal commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It is commonly confused with dark fine-grained rocks such as andesite, diabase, slag, or artificial stone.
Field identification & locations
Look for a uniformly dark, compact, fine texture and greater density than many sedimentary rocks; a streak is usually gray, and dilute acid produces no fizz. Confirming identification requires a fresh surface, hand lens, magnetism test, and locality information.
