Granodiorite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (white, grey, and black). Luster: Phaneritic (crystalline). Crystal structure: Coarse-grained granular. Cleavage: Indistinct on the rock, though constituent minerals have their own characteristic cleavages. Specific gravity: 2.6 to 2.7.

Formation & Geological History

Formed through the slow cooling of silica-rich magma deep underground (intrusive). It is typical of the roots of volcanic arcs and the margins of continents. It can date back to any geological era from the Precambrian to the Cenozoic depending on the batholith location.

Uses & Applications

Used primarily in heavy construction, road building (crushed stone), and as decorative architectural stone (dimension stone) for countertops and monuments. Often sold under the commercial name 'Granite'.

Geological Facts

Granodiorite is the rock that makes up the famous Rosetta Stone. It is chemically intermediate between granite and diorite, containing more plagioclase feldspar than orthoclase (potassium) feldspar.

Field Identification & Locations

Identify by its coarse-grained 'salt and pepper' appearance. Look for the presence of translucent quartz, white/grey plagioclase, and dark minerals like biotite or hornblende. It is extremely common in the Sierra Nevada mountains of California and other major mountain ranges.

Identified on: 4/18/2026

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