igneous
Diorite
Phaneritic Intrusive Igneous Rock (composed of Plagioclase feldspar, Hornblende, and Biotite)
Hardness (Mohs): 6-7; Color: Black and white 'salt and pepper' appearance; Luster: Sub-vitreous to dull; Crystal Structure: Phaneritic (coarse-grained, visible crystals); Specific Gravity: 2.8 - 3.0. It lacks the pinkish potassium feldspar and high quartz content of granite.
- Color
- Black and white 'salt and pepper' appearance
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness (Mohs): 6-7; Color: Black and white 'salt and pepper' appearance; Luster: Sub-vitreous to dull; Crystal Structure: Phaneritic (coarse-grained, visible crystals); Specific Gravity: 2.8 - 3.0. It lacks the pinkish potassium feldspar and high quartz content of granite.
Formation & geological history
Formed through the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain-building regions above subduction zones. Typically formed during major orogenic events throughout geological history, often found in major batholiths.
Uses & applications
Widely used as 'crushed stone' for road construction and parking lots. When polished, it is used as dimension stone for building facades, floor tiles, and more rarely as polished river stones or garden ornaments.
Geological facts
Diorite was extremely popular in Ancient Egypt for sculpting, as it is harder than stone-age tools. The famous Code of Hammurabi was inscribed on a massive stele made of a variety of black diorite.
Field identification & locations
Identify in the field by its speckled 'salt and pepper' texture; unlike granite, it contains little to no visible quartz and is darker. Found worldwide in regions with past volcanic activity like the Andes Mountains, the Alps, and the Appalachian range. For collectors, rounded river-worn specimens like this are common.
