Diorite

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Salt and pepper (mottled black and white); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Prominent on individual plagioclase crystals; Specific gravity: 2.8-3.0.

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface in volcanic arcs or cordilleran mountain building zones. It is intermediate in composition between granite and gabbro.

Uses & Applications

Used in construction as crushed stone for road bases, as a building stone (facing stone/pavers), and occasionally as a base material for statues and decorative carvings due to its durability.

Geological Facts

Ancient Egyptians often used diorite for sculpture because it is extremely hard and durable; the Code of Hammurabi is famously inscribed on a black basalt/diorite stele.

Field Identification & Locations

Identify by its 'salt and pepper' appearance caused by the mixture of light plagioclase feldspar and dark minerals like hornblende or biotite. Found in areas with deep-seated tectonic activity like the Andes or Cascades.

Identified on: 4/22/2026

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