Diorite (Gneissic)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: 'Salt and pepper' appearance (black and white/gray). Luster: Dull to sub-vitreous. Structure: Phaneritic (coarse-grained), appearing slightly foliated due to weathering or metamorphic influence. Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed by the slow cooling of mid-silica magma beneath the Earth's surface (intrusive). This specific cobble shows rounding from fluvial or glacial transport and potential metamorphic stretching (foliation). Found in volcanic arcs and mountain-building regions.

Uses & Applications

Used primarily as crushed stone for road construction, drainage, and as a durable building material (dimension stone) similar to granite. Occasionally used for carvings.

Geological Facts

Diorite is often called 'Black Granite' in the commercial trade. The famous Code of Hammurabi was inscribed on a massive stele made of a variety of black diorite.

Field Identification & Locations

Identify by its phaneritic texture with roughly equal parts dark (hornblende/biotite) and light (plagioclase feldspar) minerals. Absence of quartz distinguishes it from granite. It is commonly found in riverbeds or glacial till in mountainous regions.

Identified on: 4/21/2026

Mode: Standard