Diabase is a fine-to-medium-grained mafic intrusive rock that forms in shallow dikes and sills. It serves as the chemical equivalent to basalt but features a distinct sub-ophitic texture visible under a hand lens where laths of plagioclase are partially enclosed by pyroxene crystals.
Is this diabase?
5-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 diabase with a known reference. Diabase sits at Mohs 6-7 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Diabase leaves a white to gray streak.
- 3Read the lusterHold the specimen under a strong light. Diabase typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: dark gray, black, greenish-gray.
- 5Look at form & habitTypical habit: massive.
Often confused with
Diabase vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Diabase leaves white to gray, Basalt leaves white.
How to tell apart: Streak differs — Diabase leaves white to gray, Gabbro leaves none; luster reads dull on Diabase and vitreous on Gabbro.
How to tell apart: Luster reads dull on Diabase and subvitreous to pearly on Amphibolite.
Often found alongside diabase
Minerals reported to co-occur with diabase. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 6-7
- Density
- 2.8-3.1 g/cm³
- Streak
- White to Gray
- Luster
- Dull
- Transparency
- Opaque
- Crystal habit
- Massive
- Cleavage
- None
- Rarity
- Common
- Uses
- Construction, Road Aggregate, Decorative, Sculpture
- Host rock
- Intrusive Igneous Dikes and Sills
- Typical price
- $5-20 per specimen
Where rockhounds find diabase
1 mapped spotsClassic worldwide localities
- Palisades Sill, USA
- Scotland
- South Africa
- India
- Australia
Field-hunting tip
Look in intrusive igneous dikes and sills country — that is the host setting where diabase typically forms. If you start seeing plagioclase, pyroxene, magnetite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in California — start trip planning there.
Common questions
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Find diabase on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
