igneous

Diorite

Phaneritic Intermediate Plutonic Rock (Diorite)

Hardness: 6-7 on Mohs scale; Color: 'Salt and Pepper' appearance (mottled black and white); Luster: Dull to sub-vitreous; Crystal Structure: Phaneritic (coarse-grained); Composition: Primarily plagioclase feldspar, biotite, and hornblende.

Hardness: 6-7 on Mohs scale
Color: 'Salt and Pepper' appearance (mottled black and white)
Luster: Dull to sub-vitreous

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of magma beneath the Earth's surface in volcanic arcs and mountain-building zones (subduction zones). It is usually Mesozoic to Cenozoic in age, though it can be found in much older crustal blocks.

Uses & applications

Used primarily as crushed stone for road construction, drainage, and erosion control. High-quality specimens are polished for use as 'black granite' in countertops, floor tiles, and cemetery monuments.

Geological facts

In Ancient Egypt, diorite was so valued for its hardness and durability that it was used to carve statues of pharaohs, such as the famous statue of Khafre. The Code of Hammurabi was also famously inscribed on a diorite stele.

Field identification & locations

Identify in the field by its distinctive 'salt and pepper' look with roughly equal parts light and dark minerals and the lack of visible quartz (unlike granite). Commonly found in the Andes Mountains and the Cascades of North America.

Other igneous specimens

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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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral