igneous
Diorite (River Cobble)
Phaneritic Intermediate Plutonic Rock
Hardness: 6-7 (Mohs scale); Color: Salt-and-pepper (mottled white/gray and black); Luster: Dull to vitreous; Crystal Structure: Phaneritic (coarse-grained); Cleavage: Two directions at nearly 90 degrees (plagioclase); Specific Gravity: 2.8 - 3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Salt-and-pepper (mottled white/gray and black)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Salt-and-pepper (mottled white/gray and black); Luster: Dull to vitreous; Crystal Structure: Phaneritic (coarse-grained); Cleavage: Two directions at nearly 90 degrees (plagioclase); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the slow cooling of intermediate magma beneath the Earth's surface. Often found in volcanic arcs or orogenic belts where subduction occurs. This specimen is a cobble, meaning its rounded shape was formed by water erosion in a stream or riverbed.
Uses & applications
Used primarily in building construction as crushed stone or 'black granite' for countertops, monuments, and decorative facing stone. Smaller rounded cobbles are common in landscaping.
Geological facts
Diorite was used by ancient civilizations for sculpture and inscriptions because of its extreme hardness; the famous Code of Hammurabi was carved onto a Diorite stele.
Field identification & locations
Identify in the field by its distinctive 'salt-and-pepper' look with roughly equal parts dark and light minerals. Unlike granite, it contains little to no visible quartz. Commonly found in mountain ranges or riverbeds near igneous outcrops.
