igneous

Diorite (Polished River Stone)

Intrusive Igneous Diorite

Hardness: 6-7 Mohs; Color: Salt and pepper (mottled grey, black, and white); Luster: Sub-vitreous to dull (polished in image); Structure: Phaneritic (coarse-grained); Specific Gravity: 2.8-3.0.

Hardness: 6-7 Mohs
Color: Salt and pepper (mottled grey, black, and white)
Luster: Sub-vitreous to dull (polished in image)

Identified May 31, 2026More igneous →

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Physical properties

Hardness: 6-7 Mohs; Color: Salt and pepper (mottled grey, black, and white); Luster: Sub-vitreous to dull (polished in image); Structure: Phaneritic (coarse-grained); Specific Gravity: 2.8-3.0.

Formation & geological history

Formed from the slow cooling of silicate magma below Earth's surface (intrusive) in volcanic arcs and mountain building zones. It is intermediate in composition between granite and gabbro.

Uses & applications

Used as crushed stone for road construction, base material, and decorative landscaping. High-quality specimens are used as 'black granite' for countertops, floor tiles, and cemetery monuments.

Geological facts

Diorite was used in ancient Egypt and Mesopotamia for sculptures and stelas because of its extreme hardness; the famous Code of Hammurabi is inscribed on a diorite stele.

Field identification & locations

Identify in the field by its characteristic 'salt and pepper' appearance without visible quartz. Commonly found in roots of mountain ranges like the Andes or the Alps. It is often found as smooth cobbles in riverbeds.

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