Diorite (Salt and Pepper Rock)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Black and white ('salt and pepper' appearance); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Perfect in two directions for amphibole/feldspar components; Specific gravity: 2.8-3.0.

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain building zones (subduction zones). It typically dates back to various tectonic events throughout Earth's history.

Uses & Applications

Commonly used as a base material in construction of roads and buildings. High-quality specimens are used as structural stone, floor tiles, and occasionally in jewelry as cabochons or beads.

Geological Facts

Diorite is extremely hard and was frequently used by ancient civilizations, such as the Egyptians and Akkadians, for making statues and steles. The famous Code of Hammurabi was inscribed on a black diorite stele.

Field Identification & Locations

Identify in the field by its distinctive 'salt and pepper' look where white plagioclase feldspar mixes with dark minerals like hornblende or biotite. It lacks the pinkish potassium feldspar found in granite and the higher quartz content of granodiorite.

Identified on: 4/19/2026

Mode: Standard