Diorite

Physical Properties

Hardness: 6-7 Mohs; Color: 'Salt and Pepper' mix of black, white, and grey; Luster: Dull to sub-vitreous; Crystal Structure: Phaneritic (coarse-grained); Specific Gravity: 2.8-3.0; Cleavage: Good in two directions (due to feldspar and amphibole).

Formation & Geological History

Formed from the slow cooling of magma deep underground (intrusive). Usually found in volcanic arcs and mountain-building regions where subduction occurs. Found across various geological ages from the Precambrian to Cenozoic.

Uses & Applications

Commonly used as a base material in road construction, drainage stone, and as architectural stone (countertops, tiles, and monuments) often sold under the trade name 'black granite'.

Geological Facts

Ancient Egyptians used diorite to carve hard statues and steles, including the famous Code of Hammurabi. Its extreme hardness made it difficult for early civilizations to work with, signifies high status.

Field Identification & Locations

Identify by its interlocking 'salt and pepper' crystalline texture without the high quartz content of granite. Common in batholiths and sills in mountain ranges like the Andes or the Cascade Mountains. Collectors look for even distribution of minerals.

Identified on: 4/25/2026

Mode: Standard