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

Identified April 9, 2026More igneous →

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Physical properties

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).

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous