igneous
Diorite
Phaneritic Intrusive Igneous Rock (Intermediate Composition)
Hardness: 6-7 Mohs; Color: Salt and pepper (dull white to grey and dark green to black); Luster: Subvitreous to dull; Crystal Structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to amphibole/feldspar); Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper (dull white to grey and dark green to black)
- Luster
- Subvitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper (dull white to grey and dark green to black); Luster: Subvitreous to dull; Crystal Structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to amphibole/feldspar); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the slow cooling of magma beneath the Earth's surface in volcanic arcs or mountain-building zones. This intermediate rock type forms between granitic and basaltic compositions and is typically associated with subduction zones.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and infrastructure. High-quality specimens are polished for use as 'black granite' countertops, floor tiles, and monumental masonry.
Geological facts
Diorite was used in ancient Egypt and Mesopotamia for sculptures and inscriptions because of its extreme hardness; the Code of Hammurabi was famously carved onto a diorite stele.
Field identification & locations
Identify in the field by its distinct speckling; it contains more dark minerals than granite but lacks the quartz content. Look for it in mountainous regions or exposed batholiths. Collectors should look for balanced 'dalmatian' patterns.
