igneous
Gabbro (Water-worn pebble)
Gabbro
Hardness: 6-7 on Mohs scale. Color: Dark gray to black with speckled white/gray plagioclase crystals. Luster: Dull to phaneritic (visible crystals). Crystal structure: Coarse-grained phaneritic. Cleavage: Good in two directions (due to feldspar). Specific gravity: 2.7-3.3.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to phaneritic (visible crystals)
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark gray to black with speckled white/gray plagioclase crystals. Luster: Dull to phaneritic (visible crystals). Crystal structure: Coarse-grained phaneritic. Cleavage: Good in two directions (due to feldspar). 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 has been chemically and mechanically weathered into a smooth oval shape by water action in a river or coastal environment.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and occasionally as 'black granite' for kitchen countertops and floor tiles. Polished stones are used in landscaping and aquarium decor.
Geological facts
Gabbro is the intrusive equivalent of basalt and makes up a significant portion of the deep oceanic crust. It is often found in large plutons or sills.
Field identification & locations
Identify by its dark, coarse-grained texture and heavy weight. In the field, look for 'salt and pepper' appearances where white plagioclase is mixed with dark pyroxene. Common in mountainous regions with ancient tectonic activity.
