Diorite (River Cobble)

Physical Properties

Hardness: 6-7 on Mohs scale; Color: 'Salt and pepper' (mottled grey, black, and white); Luster: Phaneritic (crystalline) but dull when weathered; Crystal structure: Coarse-grained; Cleavage: Dependent on mineral constituents (feldspar/hornblende).

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain-building zones. This specific specimen has been rounded and smoothed by water transport (river or glacial action).

Uses & Applications

Commonly used as crushed stone for road construction, base material for driveways, and historically as a medium for sculpture and architectural stone due to its durability.

Geological Facts

Diorite is extremely hard and was used by ancient Egyptians and Mesopotamians to carve statues and steles, such as the famous Code of Hammurabi which is inscribed on a black diorite stele.

Field Identification & Locations

Identify in the field by its speckled 'salt and pepper' appearance and the lack of visible quartz compared to granite. It is commonly found in gravel pits, riverbeds, and mountainous regions with past volcanic activity.

Identified on: 4/16/2026

Mode: Standard