Diorite
Intermediate intrusive igneous rock (plagioclase + amphibole)
A coarse-grained intrusive rock with a distinctive salt-and-pepper look, the plutonic equivalent of andesite.
- Mohs hardness
- 6-7
- Color
- speckled black-and-white (salt-and-pepper), grey to greenish-grey
- Type
- igneous
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Overview
Diorite is a coarse-grained intrusive igneous rock of intermediate composition, the plutonic equivalent of andesite. It consists mainly of white-to-grey plagioclase feldspar and dark amphibole (hornblende) or pyroxene, giving it a distinctive speckled "salt-and-pepper" appearance.
Because it cools slowly underground, its crystals are large enough to see clearly. Diorite contains little or no quartz, distinguishing it from granite and granodiorite.
It is a hard, tough rock that has been prized since antiquity for sculpture and building, and is sometimes sold commercially as "black granite."
Formation & geology
Diorite forms when intermediate magma cools and crystallizes slowly deep within the crust, often in plutons, stocks, and batholiths associated with subduction-zone settings. The slow cooling allows large interlocking crystals of plagioclase and dark minerals to grow.
It commonly occurs alongside granite and granodiorite in continental magmatic arcs. When the same intermediate magma erupts and cools quickly at the surface, it forms andesite instead. Notable occurrences include continental arc batholiths worldwide.
How to identify it
Identify diorite by its coarse, visibly crystalline texture and striking black-and-white salt-and-pepper pattern of plagioclase and dark amphibole or pyroxene. Hardness is about 6-7.
It contains little or no quartz, which separates it from granite (more quartz, often pink or grey feldspar) and granodiorite (intermediate, more quartz). It is lighter and less mafic than gabbro.
The even mix of light and dark coarse grains with minimal quartz is the key diagnostic feature.
Uses & significance
Diorite is used as crushed stone for road base, concrete aggregate, and ballast. Polished, it serves as a durable dimension stone for countertops, facing, and monuments, sometimes marketed as black granite.
Historically it was highly valued; ancient civilizations carved diorite into statues and stelae, including the famous Code of Hammurabi, because of its hardness and fine polish.
It is rarely used in jewelry but is significant as a building and ornamental stone.
Frequently asked questions
What gives diorite its salt-and-pepper look?
The mix of light plagioclase feldspar and dark amphibole or pyroxene crystals creates its speckled black-and-white appearance.
Is diorite the same as granite?
No. Diorite has little or no quartz and is intermediate in composition, while granite is quartz-rich and felsic.
How is diorite related to andesite?
They share composition; diorite is the coarse-grained intrusive form, while andesite is the fine-grained volcanic equivalent.
Why was diorite used in ancient sculpture?
Its hardness and ability to take a fine polish made it prized for durable statues and monuments like the Code of Hammurabi.
Diorite guides
In-depth guides for identifying, valuing, and understanding Diorite.
Diorite identified by the community
Real specimens identified with Rock Identifier.
