Gabbro

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Dark gray to black, sometimes with green or blue tints; Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to pyroxene/plagioclase); Specific Gravity: 2.7-3.3

Formation & Geological History

Formed from the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth\'s surface (intrusive). It is the plutonic equivalent of basalt and is often found in oceanic crust and large layered intrusions.

Uses & Applications

Crushed stone for road construction, railroad ballast, concrete aggregate, and occasional use as \'black granite\' for kitchen countertops and floor tiles.

Geological Facts

Gabbro makes up a significant portion of the Earth\'s deep oceanic crust. When it contains high amounts of the mineral labradorite, it can show a beautiful iridescent play of colors known as labradorescence.

Field Identification & Locations

Identify by its dark color and coarse, interlocking crystals that are visible to the naked eye. Found in Ophiolites or large igneous provinces. Collectors look for phaneritic texture without visible quartz crystals.

Identified on: 5/4/2026

Mode: Standard