Diorite

Physical Properties

Hardness: 6-7 (Mohs scale); Color: 'Salt and pepper' (mottled black and white); Luster: Sub-vitreous to dull; Structure: Phaneritic (coarse-grained); Cleavage: Two directions (amphibole and feldspar); 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 (subduction zones). It is typically intrusive and found in batholiths and dikes.

Uses & Applications

Used primarily as a crushed stone for construction, road base, and drainage. High-quality specimens are polished for use as building facades, countertops, and floor tiles. Historically used for carving, most notably the Code of Hammurabi.

Geological Facts

Diorite is often called 'salt and pepper rock' because of its distinct mix of light plagioclase feldspar and dark minerals like hornblende. It is the plutonic equivalent of the volcanic rock andesite.

Field Identification & Locations

Identify in the field by its coarse grain and roughly equal mix of light and dark minerals. Unlike granite, it lacks significant quartz (less than 5%). Commonly found in volcanic arc settings like the Andes or the Cascades.

Identified on: 4/22/2026

Mode: Standard