Diorite

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Salt-and-pepper (black and white); Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained); Composition: Primarily plagioclase feldspar and hornblende/biotite.

Formation & Geological History

Formed through the slow cooling of intermediate magma beneath the Earth's surface in volcanic arcs and mountain-building zones (subduction zones). Can range from Precambrian to Cenozoic in age.

Uses & Applications

Used primarily in construction as crushed stone or 'black granite' for counter tops, floor tiles, and building veneers. Historically used for carving by ancient civilizations.

Geological Facts

The Code of Hammurabi was inscribed on a 7-foot tall pillar of black diorite. It is often referred to as 'Salt and Pepper' rock due to its contrasting mineral colors.

Field Identification & Locations

Identify in the field by its coarse-grained texture and near-equal distribution of dark (mafic) and light (felsic) minerals. Found in large batholiths or sills in mountainous regions like the Andes or Cascades.

Identified on: 4/21/2026

Mode: Standard