Diorite (River Cobble)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Salt and pepper (mottled white and black/dark grey); Luster: Phaneritic (crystalline); Structure: Massive, coarse-grained minerals; Cleavage: Visible in individual feldspar grains; Specific Gravity: 2.8-3.0

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface (intrusive). This specific specimen has been rounded and smoothed by water transport in a river or stream environment.

Uses & Applications

Used primarily as crushed stone for road construction, base material, and occasionally as dimension stone for buildings or ornamental 'salt and pepper' countertops.

Geological Facts

Diorite is often called the 'salt and pepper' rock because of its distinct mixture of light plagioclase feldspar and dark minerals like hornblende or biotite. It is chemically intermediate between granite and gabbro.

Field Identification & Locations

Identify it by its distinct 'salt and pepper' appearance and lack of visible quartz (unlike granite). Commonly found in volcanic arcs and mountain roots. This rounded shape indicates it was found in a high-energy water environment.

Identified on: 4/24/2026

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