Gabbro (Water-Worn Cobble)

Physical Properties

Hardness: 6-7 Mohs; Color: Dark grey, black, or dark green with white/grey plagioclase mottling; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained); 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). This specific specimen appears to be a river-worn or ocean-tumbled cobble, smoothed by mechanical erosion.

Uses & Applications

Used in construction as 'black granite' for countertops, paving stones, and as crushed stone for road base or railroad ballast.

Geological Facts

Gabbro is the chemical equivalent of basalt but cooled slowly underground rather than erupting. It makes up a significant portion of the Earth's deep oceanic crust.

Field Identification & Locations

Identify by its dark color and coarse, interlocking crystalline texture. Look for white speckles of plagioclase feldspar against a dark background of pyroxene. Common in mountainous regions or as glacial till.

Identified on: 5/3/2026

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