Diorite (Salt and Pepper Rock)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Black and white mottled; Luster: Phaneritic (crystalline/dull); Crystal Structure: Coarse-grained intrusive; Cleavage: Two directions at nearly 90 degrees; Specific Gravity: 2.8 - 3.0

Formation & Geological History

Formed by the slow cooling of silicate magma beneath the Earth's surface in volcanic arcs and mountain-building regions. Its composition is intermediate between acidic granite and basic gabbro.

Uses & Applications

Used primarily as a crushed stone for construction, road base, and drainage. High-quality specimens are polished for use as 'black granite' in kitchen countertops, commemorative plaques, and building facades.

Geological Facts

Diorite was used by ancient civilizations for sculpture and writing because of its extreme hardness; the famous Code of Hammurabi was carved into a 7.5-foot diorite stele.

Field Identification & Locations

Identify in the field by its distinctive 'salt and pepper' appearance caused by roughly equal parts white plagioclase feldspar and dark minerals like hornblende or biotite. It is found in mountain ranges such as the Andes or Cascades.

Identified on: 4/15/2026

Mode: Standard