igneous
Tumbled dark pebble, likely basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectVery dark gray to black, smooth and rounded from tumbling or water wear, with a dull to faintly silky luster; typically Mohs 5–6, dense, fine-grained, and lacking visible cleavage or crystals.
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Physical properties
Very dark gray to black, smooth and rounded from tumbling or water wear, with a dull to faintly silky luster; typically Mohs 5–6, dense, fine-grained, and lacking visible cleavage or crystals.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The specimen’s geological age cannot be determined from the image.
Uses & applications
Used as aggregate, road stone, riprap, and construction material; polished basalt is also sold as a decorative stone, massage stone, or inexpensive collectible.
Geological facts
Its compact, uniform appearance and high density are typical of basalt, though other dark rocks such as andesite, diabase, or fine-grained metamorphic rocks can look similar.
Field identification & locations
Common worldwide, especially near volcanic regions and ocean beaches; field identification is aided by black color, fine texture, heaviness, and lack of fizz with dilute acid. Exact identification requires a fresh surface or testing.
