Gabbro

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Dark grey to black, sometimes with a greenish tint. Luster: Vitreous to dull. Crystal structure: Phaneritic (coarse-grained). Cleavage: Two directions at nearly 90 degrees (pyroxene). Specific gravity: 2.7 to 3.3.

Formation & Geological History

Formed from the slow cooling and crystallization of magnesium-rich and iron-rich magma deep beneath the Earth's surface (intrusive). They occur in oceanic crust and thick continental intrusions.

Uses & Applications

Used in construction as crushed stone for road base and railroad ballast. In the dimension stone industry, it is sold as 'black granite' for countertops, floor tiles, and cemetery markers.

Geological Facts

Gabbro is the chemical equivalent of basalt, but while basalt is extrusive (fine-grained), gabbro is intrusive (coarse-grained). It makes up a significant portion of the Earth's lower oceanic crust.

Field Identification & Locations

Identify by its dark color, heavy weight (relative to granite), and visible interlocking crystals. Look for it in ophiolite complexes or deep-seated volcanic mountain ranges.

Identified on: 4/22/2026

Mode: Standard