Diorite is a coarse-grained plutonic rock primarily composed of plagioclase feldspar and dark ferromagnesian minerals like hornblende or biotite. It lacks significant quartz, which distinguishes it visually from granite by its darker, salt-and-pepper appearance. Historically prized for its hardness, it is often found in large intrusive igneous masses.
Is this diorite?
4-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch diorite with a known reference. Diorite sits at Mohs 6-7, softer than the next harder reference, harder than the previous one.
- 2Pick out the mineralsComposition: Mainly plagioclase (typically andesine), biotite and hornblende, sometimes pyroxene; under 20 percent quartz. Identify the two or three most abundant grains.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: gray, black, white.
Often confused with
Diorite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside diorite
Minerals reported to co-occur with diorite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 6-7
- Density
- 2.8-3.0 g/cm³
- Luster
- Vitreous
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Construction, Sculpture, Decorative
- Host rock
- plutonic igneous environments
- Composition
- Mainly plagioclase (typically andesine), biotite and hornblende, sometimes pyroxene; under 20 percent quartz
Where rockhounds find diorite
Classic worldwide localities
- Scotland
- Germany
- Finland
- Sweden
- United States
Field-hunting tip
Where it forms: plutonic igneous environments. If you start seeing plagioclase, biotite, hornblende in float, you are in the right ground.
Common questions
How do you identify diorite?+
Where is diorite found?+
What rocks look like diorite?+
What minerals are found with diorite?+
What kind of rock does diorite form in?+
What is diorite used for?+
Find diorite on the map
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