igneous
Diorite (River Pebble)
Intermediate plutonic igneous rock
Hardness: 6-7 Mohs; Color: Salt-and-pepper (grey, black, white); Luster: Dull (weathered) to vitreous; Structure: Phaneritic (visible grains); Composition: Plagioclase feldspar, biotite, hornblende
- Hardness
- 6-7 Mohs
- Color
- Salt-and-pepper (grey, black, white)
- Luster
- Dull (weathered) to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Salt-and-pepper (grey, black, white); Luster: Dull (weathered) to vitreous; Structure: Phaneritic (visible grains); Composition: Plagioclase feldspar, biotite, hornblende
Formation & geological history
Formed by the slow cooling of magma deep beneath the Earth's surface (intrusive). This specific specimen has been rounded and smoothed by water transport in a river or coastal environment.
Uses & applications
Commonly used as crushed stone for road construction, base material, or, in high-quality large slabs, as polished decorative stone for countertops known as 'black granite'.
Geological facts
Diorite was used in ancient Egypt and Mesopotamia for sculptures because of its extreme hardness; the famous Code of Hammurabi was inscribed on a diorite stele.
Field identification & locations
Identify by its 'salt and pepper' appearance with roughly equal parts light and dark minerals. Found in volcanic arcs and mountain building zones. Watch for the lack of quartz compared to granite.
