igneous
Gabbro
Gabbro (mafic intrusive rock)
Hardness: 6-7 Mohs; Color: dark gray to black, sometimes with green or blue hues; Luster: dull to vitreous; Crystal Structure: phaneritic (coarse-grained); Cleavage: good (from pyroxene and plagioclase content); Specific Gravity: 2.7-3.3.
- Hardness
- 6-7 Mohs
- Color
- dark gray to black, sometimes with green or blue hues
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: dark gray to black, sometimes with green or blue hues; Luster: dull to vitreous; Crystal Structure: phaneritic (coarse-grained); Cleavage: good (from pyroxene and plagioclase content); 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. It is the plutonic equivalent of basalt and is typically associated with the oceanic crust.
Uses & applications
Used in the construction industry as 'black granite' for countertops, floor tiles, and facing stone; also used as a heavy-duty crushed stone for road base and railroad ballast.
Geological facts
Gabbro makes up a large proportion of the Earth's lower oceanic crust. It is often confused with black granite in the commercial marketplace, though geologically it is much lower in silica.
Field identification & locations
Identify by its dark color and visible interlocking crystals of plagioclase and pyroxene. Found near plate boundaries and deep-seated volcanic sites. Collectors look for unusually large crystals or associated rare minerals.
