igneous
Diorite (Porphyritic)
Porphyritic Diorite
Hardness: 6-7 (Mohs scale); Color: 'Salt and pepper' (dark grey matrix with white phenocrysts); Luster: Phaneritic/Dull to sub-vitreous; Crystal Structure: Medium to coarse-grained crystals; Cleavage: Good (in feldspar components); Specific Gravity: 2.8 - 3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- 'Salt and pepper' (dark grey matrix with white phenocrysts)
- Luster
- Phaneritic/Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: 'Salt and pepper' (dark grey matrix with white phenocrysts); Luster: Phaneritic/Dull to sub-vitreous; Crystal Structure: Medium to coarse-grained crystals; Cleavage: Good (in feldspar components); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed through the slow cooling of intermediate magma deep within the earth's crust. This specific specimen shows porphyritic texture, indicating two stages of cooling: a slow stage forming the white phenocrysts and a faster stage forming the dark matrix.
Uses & applications
Used in construction as crushed stone for roads, countertops (often sold as 'black granite'), architectural facing, and as ornamental garden stones.
Geological facts
Diorite was used in ancient Egypt and Mesopotamia for sculptures and tablets because of its extreme hardness; the famous Code of Hammurabi was inscribed on a diorite stele.
Field identification & locations
Identify in the field by its speckled 'salt and pepper' appearance and lack of visible quartz compared to granite. It is commonly found in volcanic arcs and mountain building zones (orogenic belts).
