igneous
Diorite (Gneissic)
Diorite with possible metamorphic foliation
Hardness: 6-7 Mohs; Color: Salt and pepper (black and white); Luster: Dull to sub-vitreous; Structure: Phaneritic (visible crystals) with slight foliation; Composition: Plagioclase feldspar and hornblende/biotite; SG: 2.8-3.0
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper (black and white)
- Luster
- Dull to sub-vitreous
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper (black and white); Luster: Dull to sub-vitreous; Structure: Phaneritic (visible crystals) with slight foliation; Composition: Plagioclase feldspar and hornblende/biotite; SG: 2.8-3.0
Formation & geological history
Formed from the slow cooling of intermediate magma beneath the Earth's surface (igneous). The slight banding suggests it may have undergone some heat and pressure (metamorphism) or flow during cooling.
Uses & applications
Used in construction as crushed stone, floor tiles, and architectural veneers. High-quality specimens are polished for decorative use.
Geological facts
Diorite is often called 'Black Granite' in the commercial stone trade. It was used in Ancient Egypt for sculptures and the Code of Hammurabi was famously inscribed on a diorite stele.
Field identification & locations
Identify by its 'salt and pepper' appearance and lack of visible quartz compared to granite. Found in volcanic arcs and mountain-building regions. Common in the Andes Mountains and parts of the UK and North America.
