igneous
Likely gabbro or diabase
Mafic igneous rock, chiefly plagioclase feldspar and pyroxene; possible olivine and amphibole
AI-generated identification · may be incorrectDark gray-green, coarse granular texture with dull to weakly vitreous luster; hardness about 6–7, no obvious cleavage as a rock, and typical specific gravity 2.8–3.1. The image is insufficient for definitive identification.
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Physical properties
Dark gray-green, coarse granular texture with dull to weakly vitreous luster; hardness about 6–7, no obvious cleavage as a rock, and typical specific gravity 2.8–3.1. The image is insufficient for definitive identification.
Formation & geological history
Forms when iron- and magnesium-rich magma cools slowly underground (gabbro) or in shallow dikes (diabase/dolerite), generally in Precambrian to Phanerozoic intrusive bodies. It commonly crystallizes from basaltic magma.
Uses & applications
Used as crushed aggregate, road stone, riprap, and sometimes dimension stone; attractive polished pieces may be sold as decorative stone, but this fragment has little gem value.
Geological facts
Its dark color reflects abundant mafic minerals, while pale specks are likely plagioclase. A streak, magnetism test, fresh surface, and hand lens would help distinguish it from metamorphic amphibolite or serpentinite.
Field identification & locations
Common in oceanic crust, mafic intrusions, and areas such as the Appalachian and Canadian Shield regions. A granular interlocking texture, high density, and white feldspar mixed with dark pyroxene support gabbro/diabase.
