Gabbro

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark green, gray, or black; Luster: Phaneritic (coarse-grained) and dull to vitreous; Crystal Structure: Predominantly plagioclase feldspar and pyroxene (augite); Specific gravity: 2.7-3.3.

Formation & Geological History

Formed by the slow crystallization of magnesium-rich and iron-rich magma beneath the Earth's surface. It is the intrusive equivalent of basalt and is often found in deep oceanic crust or within large continental intrusions.

Uses & Applications

Commonly used as crushed stone for road construction, railroad ballast, and as a concrete aggregate. High-quality varieties are polished and sold as 'Black Granite' for countertops and floor tiles.

Geological Facts

Gabbro makes up a large portion of the Earth's lower oceanic crust. It is often associated with valuable ore deposits of nickel, chromium, and platinum.

Field Identification & Locations

Identify by its dark color and coarse, interlocking crystals that are visible to the naked eye. It lacks the quartz found in granite. Common in the deep roots of mountains or exposed in ophiolite complexes.

Identified on: 5/8/2026

Mode: Standard