igneous
Diorite with Weathering Rind
Course-grained intermediate plutonic rock (Diorite)
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (white plagioclase, black hornblende/biotite) on fresh surfaces; tan/brown on weathered surfaces. Luster: Dull to sub-vitreous. Texture: Phaneritic (coarse-grained).
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (white plagioclase, black hornblende/biotite) on fresh surfaces; tan/brown on weathered surfaces. Luster: Dull to sub-vitreous. Texture: Phaneritic (coarse-grained).
Formation & geological history
Formed by the slow cooling of intermediate-composition magma deep within the Earth's crust (plutonic). This specific specimen shows a thick weathering rind, suggesting it spent a long time exposed to surface elements.
Uses & applications
Used primarily as crushed stone for road construction and as dimension stone for buildings, monuments, and countertops. Polished diorite is often sold as 'black granite'.
Geological facts
Diorite was used in ancient Egypt and Mesopotamia for sculptures because of its extreme hardness; the Code of Hammurabi was famously inscribed on a diorite stele.
Field identification & locations
Identify in the field by its distinctive 'spotted' appearance and lack of visible quartz (unlike granite). Commonly found in volcanic arcs and mountain roots along subduction zones.
