Igneous

Diorite (River Cobble)

Intermediate Phaneritic Plutonic Rock

Hardness: 6-7 (Mohs scale); Color: 'Salt and pepper' (mottled grey, white, and black); Luster: Phaneritic (visible interlocking crystals); Crystal structure: Coarse-grained; Specific Gravity: 2.8-3.0.

Hardness: 6-7 (Mohs scale)
Color: 'Salt and pepper' (mottled grey, white, and black)
Luster: Phaneritic (visible interlocking crystals)

Identified March 17, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow cooling of magma beneath the Earth's surface (intrusive), typically in volcanic arcs or orogenic belts. This specific specimen has been rounded by fluvial (river) or glacial transport over thousands of years.

Uses & applications

Commonly used as crushed stone for road base, drainage, and construction. Polished varieties are used for countertops, floor tiles, and dimension stone. Historically used by ancient civilizations (like the Egyptians) for durable sculptures.

Geological facts

The Code of Hammurabi, one of the oldest deciphered writings of significant length in the world, was inscribed on a 7-foot tall black diorite stele.

Field identification & locations

Identify in the field by its distinctive 'salt and pepper' look, lack of visible quartz (unlike granite), and high durability. Commonly found in mountainous regions or as river rocks in glacial till areas.

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Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

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