Diorite

Physical Properties

Hardness: 6-7 on Mohes scale; Color: Black and white ('salt and pepper' appearance); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Cleavage: Good in two directions (due to amphibole and plagioclase); 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 found in plutons or batholiths and can range from Precambrian to Cenozoic in age.

Uses & Applications

Used primarily as a crushed stone for construction, road base, and drainage. High-quality specimens are polished for use as architectural stone, countertops, and floor tiles. Historically used by ancient civilizations like the Egyptians for statues and carvings.

Geological Facts

Because of its extreme hardness, diorite was used in ancient Egypt and Mesopotamia to create durable monuments. The Code of Hammurabi was famously inscribed on a large diorite stele over 7 feet tall.

Field Identification & Locations

Identify in the field by its speckled 'salt and pepper' look, lack of visible quartz (unlike granite), and coarse crystalline texture. It is commonly found in the roots of mountain ranges such as the Andes, the Cascades, and the Alps.

Identified on: 4/22/2026

Mode: Standard