igneous
Polished dark mafic river stone
Likely basalt or dolerite (diabase), composed mainly of plagioclase feldspar and pyroxene
AI-generated identification · may be incorrectFine-grained, dark gray to black, with a subdued polished luster and tiny pale mineral grains; typically Mohs 6–7, massive, with no visible cleavage or crystal faces. Specific gravity is approximately 2.8–3.1.
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Physical properties
Fine-grained, dark gray to black, with a subdued polished luster and tiny pale mineral grains; typically Mohs 6–7, massive, with no visible cleavage or crystal faces. Specific gravity is approximately 2.8–3.1.
Formation & geological history
Formed when iron- and magnesium-rich magma cooled, either rapidly as basalt or slowly underground as dolerite. Such stones commonly weather from Mesozoic or younger volcanic rocks and become rounded through river or beach abrasion.
Uses & applications
Used as a decorative, tumbled, or display stone; unpolished basalt and dolerite are also used for aggregate, roadstone, riprap, and paving. Common specimens generally have little commercial value.
Geological facts
Its dense, uniform appearance and fine grain are characteristic of mafic volcanic rock. The smooth oval shape indicates natural water abrasion or mechanical tumbling and polishing.
Field identification & locations
Identify by its dark color, fine grain, high density, lack of layering, and inability to scratch easily with a steel knife; acid produces no reaction. Common worldwide in volcanic regions, river gravels, and beaches.
