Diorite

Physical Properties

Hardness: 6-7 Mohs; Color: Salt and pepper appearance (mottled black and white); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (medium to coarse-grained), typically containing plagioclase feldspar, hornblende, and biotite; Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed by the slow cooling of intermediate-composition magma deep beneath the Earth's surface (intrusive). Often found in volcanic arcs and mountain-building regions above subduction zones.

Uses & Applications

Commonly used as crushed stone for road construction and drainage. High-quality specimens are polished and used as 'black granite' for countertops, floor tiles, and building veneers.

Geological Facts

Diorite was used by ancient civilizations for sculpture and masonry because of its extreme hardness; the famous Code of Hammurabi was inscribed on a diorite stele.

Field Identification & Locations

Identify in the field by its distinctive 'chunky' speckled texture and lack of visible quartz (unlike granite). It is commonly found in the roots of mountain ranges like the Andes or the Cascades.

Identified on: 4/7/2026

Mode: Standard