Diorite Identification Guide
Identify diorite, the salt-and-pepper intermediate plutonic rock, by its coarse texture, mineralogy, hardness, and granite/gabbro look-alikes.
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What Diorite Looks Like
Diorite is a coarse-grained intrusive (plutonic) igneous rock of intermediate composition. Its signature is a 'salt-and-pepper' appearance: white to gray plagioclase feldspar speckled with black hornblende and/or biotite (and minor pyroxene). It contains little or no quartz and little potassium feldspar, so it lacks the pink and abundant clear-gray feldspar of granite.
Key visual cues:
- Color: overall medium gray, strongly black-and-white speckled.
- Grain size: coarse — individual crystals clearly visible.
- Texture: interlocking, equigranular, no layering or foliation.
- Minerals: white plagioclase + black hornblende/biotite; little quartz.
Step-by-Step Field-ID Checklist
- Confirm coarse, interlocking crystals — a plutonic texture, not fine like andesite.
- Identify the salt-and-pepper mix of white feldspar and black mafic minerals.
- Check for absence of quartz — few or no glassy gray grains; little pink feldspar.
- Test hardness. Constituent silicates scratch glass (Mohs ~6).
- Heft it — moderately dense, heavier than granite but lighter than gabbro.
Diagnostic Tests
- Mohs hardness: ~6 overall (feldspar/hornblende); scratches glass.
- Streak: gray, weak.
- Fracture: tough, breaks across grains; no rock cleavage.
- Density: ~2.8–3.0 g/cm³ (between granite ~2.65 and gabbro ~3.0).
- Acid: inert to dilute HCl.
- Magnetism: weak if magnetite is present.
Common Look-Alikes and How to Tell Them Apart
- Granite: lighter overall, contains abundant quartz (glassy gray grains) and often pink K-feldspar; diorite has little/no quartz and a more strictly black-and-white look.
- Gabbro: darker, more mafic (dominated by pyroxene and calcium-rich plagioclase), denser; diorite is lighter gray with hornblende and sodic-intermediate plagioclase.
- Granodiorite: intermediate between granite and diorite — has visible quartz; diorite has minimal quartz.
- Andesite: same composition but fine-grained (volcanic equivalent); diorite is coarse-grained.
- Syenite: lacks quartz like diorite but is dominated by alkali feldspar and is lighter/pinker; diorite is plagioclase-rich and grayer.
Where It Is Found
Diorite forms from intermediate magma cooling slowly at depth, typically in batholiths, stocks, and plutons along continental margins and volcanic arcs. It is common in mountain belts worldwide — the Andes, the Sierra Nevada and other Cordilleran batholiths, and Precambrian shields. Prized for its toughness, it has been quarried since antiquity for monuments and building stone.
Frequently asked questions
How can you tell if it's diorite?
Look for a coarse-grained, salt-and-pepper plutonic rock of white plagioclase and black hornblende/biotite with little or no quartz and no pink potassium feldspar, hardness ~6 that scratches glass, and density between granite and gabbro.
What is the difference between diorite and granite?
Granite contains abundant quartz and often pink potassium feldspar, giving it a lighter, more varied color, while diorite has little quartz and a stark black-and-white speckled look.
Diorite vs gabbro — how do I tell them apart?
Gabbro is darker and denser, dominated by pyroxene and calcium-rich plagioclase, while diorite is lighter gray with hornblende and intermediate plagioclase. Color and density separate them.
Is diorite the same as andesite?
They share composition, but diorite is the coarse-grained rock that cooled slowly underground, while andesite is its fine-grained volcanic equivalent that cooled fast at the surface.
Diorite identified by the community
Recent Diorite specimens identified with Rock Identifier.
