igneous

Gabbro

Gabbro (Mg, Fe)2Si2O6 / (Ca, Na)(Al, Si)4O8

Hardness: 6-7 Mohs; Color: Dark grey, black, or dark green; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Two directions at nearly 90 degrees; Specific gravity: 2.7-3.3

Hardness: 6-7 Mohs
Color: Dark grey, black, or dark green
Luster: Vitreous to dull

Identified May 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the slow cooling of magnesium-rich and iron-rich magma inside the Earth's crust (intrusive). They are common in the oceanic crust and often associated with mid-ocean ridges.

Uses & applications

Used extensively in the construction industry as crushed stone for road base, railroad ballast, and concrete aggregate. If polished, it is sold as 'Black Granite' for countertops and cemetery markers.

Geological facts

Gabbro is the intrusive equivalent of basalt. Large parts of the Earth's oceanic crust are made of gabbro, and it has also been identified on the surface of the Moon.

Field identification & locations

Identified in the field by its dark color and lacks quartz. It has large, interlocking crystals indicative of underground cooling. Common in areas of ancient volcanic activity and magmatic intrusions.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous