igneous
Diorite
Diorite (Intermediate Plutonic Rock)
Hardness: 6-7 (Mohs); Color: "Salt and pepper" appearance with black, grey, and white; Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarsegrained) with equigranular crystals; Specific gravity: 2.8-3.0.
- Hardness
- 6-7 (Mohs)
- Color
- "Salt and pepper" appearance with black, grey, and white
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: "Salt and pepper" appearance with black, grey, and white; Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarsegrained) with equigranular crystals; 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 mountain-building zones above subduction zones. Typically formed during major orogenic events throughout geological history.
Uses & applications
Used primarily as crushed stone for road construction and drainage. High-quality specimens are used in architectural surfacing (granite-like tiles), countertops, and occasionally for sculpting or as paving stones.
Geological facts
Diorite was used in ancient Egypt and Mesopotamia for statues and inscriptions because of its extreme hardness; the Code of Hammurabi was famously inscribed on a pillar of diorite. It is often referred to as 'Black Granite' in the commercial stone trade.
Field identification & locations
Identify in the field by its distinctive black-and-white speckled appearance and the lack of visible quartz (unlike granite). It is commonly found in large plutons or sills in the roots of mountain ranges like the Andes or the Cascade Mountains.
