Diorite

Physical Properties

Hardness: 6-7 (Mohs scale); Color: 'Salt and pepper' mixture of white, gray, and black; Luster: Dull to phaneritic sub-vitreous; Crystal structure: Phaneritic (coarse-grained) with interlocking crystals; Cleavage: Good (due to plagioclase/hornblende minerals); Specific gravity: 2.8 - 3.0.

Formation & Geological History

Formed by the slow cooling of magma deep underground (intrusive) in subduction zones above oceanic plates. Typically found in volcanic arcs and mountain building areas (orogenic belts). Range in age from Precambrian to Cenozoic.

Uses & Applications

Used primarily as crushed stone for road construction and drainage. High-quality specimens are polished for use as dimension stone, floor tiles, and grave markers (often marketed as 'Black Granite').

Geological Facts

Diorite was used in ancient Egypt and Mesopotamia to carve hard stone statues because of its durability. The famous Code of Hammurabi was inscribed on a large diorite stela.

Field Identification & Locations

Identify in the field by its characteristic black-and-white speckled appearance and medium-to-coarse grain size. Look for the presence of sodium-rich plagioclase and dark minerals like hornblende or biotite. It is found in large plutons and batholiths in mountain ranges like the Andes or Cascade mountains.

Identified on: 4/14/2026

Mode: Standard