Igneous

Gabbro

Gabbro (phaneritic mafic intrusive rock)

Hardness: 6-7 Mohs; Color: Dark grey to black, or greenish-black; Luster: Vitreous to dull; Structure: Phaneritic (coarse-grained); Cleavage: Prominent in pyroxene and plagioclase grains; Specific Gravity: 2.7 - 3.3.

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

Identified January 13, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface (plutonic). It is typically found in the lower oceanic crust and in large continental intrusions.

Uses & applications

Commonly used as crushed stone for road construction, railroad ballast, and concrete aggregate. When polished, it is sold as 'Black Granite' for use in countertops, floor tiles, and cemetery markers.

Geological facts

Gabbro is the intrusive equivalent of basalt. It is a major constituent of the oceanic crust and is often found in ophiolites, which are sections of oceanic crust thrust onto continental plates.

Field identification & locations

Identify in the field by its dark color and visible interlocking crystals of dark pyroxene and light-colored plagioclase feldspar. Common locations include the Duluth Complex in Minnesota, the Bushveld Complex in South Africa, and the Isle of Skye in Scotland. It lacks the quartz found in granite.

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