Diorite

Physical Properties

Hardness: 6-7 on Mohs scale; Color: 'Salt and pepper' (mottled black and white); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Distinct in feldspar/amphibole grains; Specific gravity: 2.8-3.0.

Formation & Geological History

Formed from the slow cooling of silicate magma below the Earth's surface in volcanic arcs and mountain-building zones (subduction zones). It is intermediate in composition between granite and gabbro.

Uses & Applications

Used primarily as crushed stone for road construction, drainage, and erosion control. High-quality specimens are polished for use as 'black granite' in countertops, floor tiles, and building veneers.

Geological Facts

Diorite is extremely tough and was favored by ancient civilizations, such as the Egyptians and Akkadians, for making durable statues and inscriptions, including the famous Code of Hammurabi stele.

Field Identification & Locations

Identify by its characteristic speckled appearance where dark minerals (biotite/hornblende) and light minerals (plagioclase feldspar) are present in roughly equal proportions. Found in large plutons or sills often associated with granite batholiths.

Identified on: 4/26/2026

Mode: Standard