igneous
Polished orbicular gabbro (likely 'black granite')
Gabbro, a coarse-grained mafic intrusive rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and amphibole
AI-generated identification · may be incorrectDark gray to black with pale, rounded or mottled mineral aggregates and a polished luster; typically massive, nonfoliated, and granular. Hardness is about 6–7 Mohs, with no obvious cleavage as a whole rock and specific gravity around 2.9–3.1.
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Physical properties
Dark gray to black with pale, rounded or mottled mineral aggregates and a polished luster; typically massive, nonfoliated, and granular. Hardness is about 6–7 Mohs, with no obvious cleavage as a whole rock and specific gravity around 2.9–3.1.
Formation & geological history
Formed when mafic magma cooled slowly deep underground, allowing large crystals to develop; most gabbro is Precambrian to Phanerozoic in age. The rounded patterns may reflect orbicular textures or alteration around mineral clusters.
Uses & applications
Used as polished decorative stone, countertops, tiles, monuments, and lapidary objects; attractive pieces are sold as display stones or worry stones.
Geological facts
Commercially labeled “black granite,” gabbro is not true granite because it is richer in iron, magnesium, and calcium and poorer in quartz.
Field identification & locations
Identify it by its high density, dark color, interlocking crystalline texture, and resistance to scratching by a steel knife. Common sources include Norway, India, Madagascar, South Africa, and many oceanic crust complexes.
