igneous
Polished dark mafic rock, likely basalt or gabbro
Basalt–gabbroic rock; mainly plagioclase feldspar, pyroxene, and possibly olivine/amphibole
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; dark gray to black with subtle bronze-gray mineral grains, subvitreous to dull luster, fine-to-medium crystalline texture, and no obvious cleavage on the rounded surface. The smooth shape indicates tumbling or water abrasion.
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Physical properties
Hardness about 5.5–6.5 Mohs; dark gray to black with subtle bronze-gray mineral grains, subvitreous to dull luster, fine-to-medium crystalline texture, and no obvious cleavage on the rounded surface. The smooth shape indicates tumbling or water abrasion.
Formation & geological history
Formed from iron- and magnesium-rich magma: basalt cools rapidly at or near the surface, whereas gabbro crystallizes slowly underground; ages vary widely from Precambrian to modern volcanic settings.
Uses & applications
Commonly used as aggregate, roadstone, riprap, ballast, and decorative stone; attractive polished pieces are inexpensive collector specimens.
Geological facts
Its dark color and visible interlocking grains are characteristic of mafic igneous material. A magnet may show weak attraction if magnetite is present.
Field identification & locations
Look for dense black-gray color, fine or interlocking crystals, high specific gravity, and absence of fizz with dilute acid; common worldwide in volcanic regions and oceanic crust. Exact identification requires fresh surface examination or microscopy.
