igneous
Polished basalt or gabbro pebble
Basaltic to gabbroic mafic rock; chiefly plagioclase feldspar and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectDark greenish-gray, fine- to medium-grained, with a subdued polished luster and scattered reflective crystals; typical hardness is about 6–7 Mohs, with no obvious cleavage as a whole rock and specific gravity roughly 2.8–3.1.
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Physical properties
Dark greenish-gray, fine- to medium-grained, with a subdued polished luster and scattered reflective crystals; typical hardness is about 6–7 Mohs, with no obvious cleavage as a whole rock and specific gravity roughly 2.8–3.1.
Formation & geological history
An igneous rock formed from iron- and magnesium-rich magma; basalt crystallizes at or near the surface, while gabbro is its coarse-grained underground equivalent. The rounded, smooth form indicates transport and abrasion by water or glacial action, likely much younger than its original crystallization age.
Uses & applications
Used as aggregate, road stone, riprap, landscaping material, and sometimes as polished decorative stone or jewelry cabochons.
Geological facts
Its dark color reflects abundant mafic minerals and iron oxides. A magnet may show weak attraction if magnetite is present; acid produces no reaction.
Field identification & locations
Common in volcanic terrains, riverbeds, beaches, and glacial deposits worldwide. Identify by its dark color, granular texture, high density, lack of fizzing, and possible weak magnetism; exact identification requires a thin section or mineral testing.
