igneous
Gabbro (or Basalt Pebble)
Mafic Intrusive/Extrusive Igneous Rock
Hardness: 5.5-6 on Mohs scale; Color: Dark grey to black with greenish/yellowish plagioclase or pyroxene mottling; Luster: Dull to sub-vitreous; Structure: Phaneritic to fine-grained crystalline; Cleavage: Variable; Specific Gravity: 2.8-3.1
- Hardness
- 5
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 5.5-6 on Mohs scale; Color: Dark grey to black with greenish/yellowish plagioclase or pyroxene mottling; Luster: Dull to sub-vitreous; Structure: Phaneritic to fine-grained crystalline; Cleavage: Variable; Specific Gravity: 2.8-3.1
Formation & geological history
Formed by the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface or as part of oceanic crustal formations.
Uses & applications
Used as crushed stone for concrete aggregate, road base material, railroad ballast, dimension stone, and as decorative landscape gravel.
Geological facts
Gabbro makes up much of the deep oceanic crust under the seafloor sediments. Highly weathered or stream-rounded pebbles often exhibit mottled tan, cream, or greenish minerals against a dark background.
Field identification & locations
Identified in the field by its dark color, heavy feel relative to size, and mottled crystalline texture. Commonly found in ancient volcanic terranes, riverbeds, beach gravels, and glaciated areas.
