igneous
Diorite
Diorite (plagioclase, hornblende, pyroxene, biotite)
Phaneritic (coarse-grained) texture; Salt-and-pepper appearance; Hardness: 6-7 on Mohs scale; Luster: Dull to vitreous; Color: Speckled black and white/grey; Specific gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Speckled black and white/grey
- Luster
- Dull to vitreous
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Physical properties
Phaneritic (coarse-grained) texture; Salt-and-pepper appearance; Hardness: 6-7 on Mohs scale; Luster: Dull to vitreous; Color: Speckled black and white/grey; Specific gravity: 2.8-3.0.
Formation & geological history
Formed through the slow cooling of magma beneath the Earth's surface (intrusive/plutonic). It usually occurs in volcanic arcs and associated with mountain building near subduction zones. Typically Mesozoic to Cenozoic in age depending on locality.
Uses & applications
Used primarily as a crushed stone for construction, road base, and drainage. In its polished form, it is used for countertops, memorials, and decorative tiles, often sold as 'black granite'.
Geological facts
The famous Code of Hammurabi was inscribed on a large black diorite stele. In Ancient Egypt, it was prized for its hardness and used to make durable sculptures and bowls.
Field identification & locations
Identify in the field by its interlocking coarse grains and distinct white (plagioclase) and black (hornblende/biotite) speckled pattern; unlike granite, it contains little to no visible quartz. Found in mountainous or volcanic regions like the Andes, Cascades, and Alps.
