Diorite (River Cobble)

Physical Properties

Hardness: 6-7 Mohs; Color: Salt-and-pepper (mottled black and white); Luster: Dull to sub-vitreous; Structure: Phaneritic (medium to coarse grained), crystalline; Specific gravity: 2.8 - 3.0.

Formation & Geological History

Formed by the slow cooling of intermediate-composition magma deep beneath the Earth's surface (plutonic). This specific specimen has been rounded by fluvial action (river erosion) over thousands of years.

Uses & Applications

Used in construction as crushed stone, road base, or building facades. Polished varieties are used for countertops and floor tiles, often sold as 'black granite'.

Geological Facts

Diorite was used by ancient civilizations for sculpture because of its extreme hardness; the Code of Hammurabi was famously inscribed on a diorite stele.

Field Identification & Locations

Identify in the field by its speckled 'salt and pepper' appearance, lack of visible quartz (distinguishing it from granite), and smooth water-worn surface if found in riverbeds. Common in mountain roots or volcanic arcs.

Identified on: 4/22/2026

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