Igneous

Diorite

Intermediate plutonic igneous rock

Hardness: 6-7 (Mohs); Color: Salt and pepper (mottled grey, white, and black); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarsegrained); Specific gravity: 2.8-3.0.

Hardness: 6-7 (Mohs)
Color: Salt and pepper (mottled grey, white, and black)
Luster: Sub-vitreous to dull

Identified April 11, 2026More igneous →

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Physical properties

Hardness: 6-7 (Mohs); Color: Salt and pepper (mottled grey, white, and black); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarsegrained); Specific gravity: 2.8-3.0.

Formation & geological history

Formed by the slow cooling of silicate magma below the Earth's surface (intrusive) in volcanic arcs and at subduction zones. Typically associated with mountain-building processes.

Uses & applications

Used primarily as a crushed stone for road construction, drainage, and concrete. High-quality specimens are polished for use as dimension stone, countertops, and floor tiles (often sold as 'black granite').

Geological facts

Ancient Egyptians and Mayans used diorite for sculptures and hieroglyphics because of its extreme hardness; the Code of Hammurabi was inscribed on a large diorite stele.

Field identification & locations

Identify in the field by its characteristic 'salt and pepper' look with relatively even amounts of light and dark minerals. Common in the Andes Mountains and parts of the UK. Check for lack of quartz to distinguish from granite.

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous