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.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Diabasic (interstitial); chilled margins fine-grained to aphanitic.
- 3Pick out the mineralsComposition: Calcium-rich plagioclase, most commonly labradorite; accessory hornblende, biotite and apatite. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull.
- 5Match the color rangeCompare against the expected color range: dark gray, black, greenish-gray.
Often confused with
Diabase vs. its common look-alikes, and how to tell them apart in the field.
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
- Luster
- Dull
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Construction, Road aggregate, Decorative, Sculpture
- Host rock
- intrusive igneous dikes and sills
- Texture
- Diabasic (interstitial); chilled margins fine-grained to aphanitic
- Composition
- Calcium-rich plagioclase, most commonly labradorite; accessory hornblende, biotite and apatite
Where rockhounds find diabase
1 mapped spotsClassic worldwide localities
- Palisades Sill, USA
- Scotland
- South Africa
- India
- Australia
Field-hunting tip
Where it forms: intrusive igneous dikes and sills. If you start seeing plagioclase, pyroxene, magnetite in float, you are in the right ground. In the U.S., the densest reported localities are in California. Start trip planning there.
Common questions
How do you identify diabase?+
Where is diabase found?+
Can I find diabase in the United States?+
What rocks look like diabase?+
What minerals are found with diabase?+
What kind of rock does diabase form in?+
What is diabase used for?+
Find diabase on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
