igneous
Diorite
Diorite (Intermediate intrusive igneous rock)
Hardness: 6-7 on Mohs scale; Color: 'Salt and pepper' (mottled black and white); Luster: Phaneritic (crystalline) and somewhat vitreous; Crystal Structure: Coarse-grained; Cleavage: Good in two directions (due to feldspar and hornblende); Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- 'Salt and pepper' (mottled black and white)
- Luster
- Phaneritic (crystalline) and somewhat vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: 'Salt and pepper' (mottled black and white); Luster: Phaneritic (crystalline) and somewhat vitreous; Crystal Structure: Coarse-grained; Cleavage: Good in two directions (due to feldspar and hornblende); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed by the slow cooling of magma beneath the Earth's surface in volcanic arcs and at subduction zones. This process creates its coarse-grained texture. These rocks can date back to various geological eras, commonly formed during mountain-building events.
Uses & applications
Used in the construction industry as 'crushed stone' for road base and drainage. Higher quality specimens are used for floor tiles, counter tops, and as a dimension stone for buildings and monuments (often sold under the trade name 'black granite').
Geological facts
Diorite is often used by ancient civilizations for stone carvings because of its hardness. The famous 'Code of Hammurabi' was inscribed on a 7-foot tall pillar of black diorite.
Field identification & locations
Identify it by its phaneritic texture and even distribution of light (plagioclase) and dark (hornblende/biotite) minerals. Found in large plutons or sills. Collectors should look for balanced 'dalmatian' patterning.
