Chalk is a soft, porous, white form of limestone composed primarily of the mineral calcite from the shells of microscopic marine organisms. It is typically found in thick marine deposits and is characterized by its earthy feel and ability to leave a white mark on surfaces. It is a common educational tool for identifying carbonate rocks through simple acid testing.
Is this chalk?
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 chalk with a known reference. Chalk sits at Mohs 1, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Fine-textured, earthy, soft and highly porous. Grain size: Mostly calcite plates 0.5 to 4 microns.
- 3Pick out the mineralsComposition: Almost pure calcite from plankton such as foraminifera and coccolithophores, with 2 to 4 percent quartz and clay minerals. 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: white, off-white, gray.
Often confused with
Chalk vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads dull on Chalk and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.
How to tell apart: Kaolinite is the harder of the two (Mohs 2-2.5 vs. 1); luster reads dull on Chalk and pearly to dull earthy on Kaolinite.
Often found alongside chalk
Minerals reported to co-occur with chalk. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 1
- Density
- 1.5-2.0 g/cm³
- Luster
- Dull
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Educational, Industrial, Decorative
- Host rock
- marine sedimentary basins
- Texture
- Fine-textured, earthy, soft and highly porous
- Grain size
- Mostly calcite plates 0.5 to 4 microns
- Composition
- Almost pure calcite from plankton such as foraminifera and coccolithophores, with 2 to 4 percent quartz and clay minerals
Where rockhounds find chalk
3 mapped spotsClassic worldwide localities
- United Kingdom
- France
- Denmark
- United States
Field-hunting tip
Where it forms: marine sedimentary basins. If you start seeing flint, chert, calcite in float, you are in the right ground. In the U.S., the densest reported localities are in Kansas, Florida. Start trip planning there.
More chalk photos
Common questions
How do you identify chalk?+
Where is chalk found?+
Can I find chalk in the United States?+
What rocks look like chalk?+
What minerals are found with chalk?+
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What is chalk used for?+
Find chalk on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
