Chalk
Calcium carbonate (CaCO3, calcite)
A soft, white, fine-grained limestone made almost entirely of microscopic marine plankton skeletons.
- Mohs hardness
- 1-3
- Color
- White to off-white, sometimes pale gray or yellowish
- Type
- sedimentary
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Overview
Chalk is a soft, white, fine-grained variety of limestone composed almost entirely of calcium carbonate (calcite). It forms from the microscopic calcite plates (coccoliths) of single-celled marine algae, along with foraminifera and other plankton.
It is famous for its brilliant white color and porous, friable texture — it crumbles easily and leaves a powdery mark, which is why "chalk" became a household word (though modern blackboard chalk is usually gypsum).
The iconic White Cliffs of Dover and the chalk downs of England are made of thick Cretaceous-age chalk; in fact the word "Cretaceous" derives from creta, Latin for chalk.
Formation & geology
Chalk forms on the floor of warm, shallow, clear seas where vast blooms of calcite-shelled plankton (coccolithophores) live in the surface waters.
When these organisms die, their tiny skeletons rain down and accumulate as a fine carbonate ooze. Over millions of years, slow burial and gentle compaction turn the ooze into chalk, with relatively little of the strong cementation that hardens ordinary limestone.
Most of the world's great chalk deposits formed during the Cretaceous Period (about 145-66 million years ago), when sea levels were high and continents were flooded by shallow seas teeming with plankton.
How to identify it
Chalk is very soft (Mohs ~1-3), white, and powdery — it can be scratched with a fingernail and will leave a white mark on your hands or a darker surface.
Like all carbonate rocks, it fizzes vigorously in dilute hydrochloric acid (or even vinegar), confirming calcite. The texture is fine, porous, and earthy.
Look-alikes: Other limestones are harder and denser. Diatomite and kaolin clay are also soft and white but do not fizz in acid (diatomite is silica). Gypsum is soft too but does not react with acid and feels slightly different.
Uses & significance
Chalk has been used for writing and drawing, as agricultural lime to reduce soil acidity, and as a raw material for cement and quicklime. Finely ground chalk (whiting) is a filler and pigment in paints, putty, toothpaste, ceramics, and rubber.
It is also an important aquifer rock — porous chalk stores and yields large volumes of groundwater in regions like southern England.
Chalk is not used in jewelry because it is far too soft, but its scenic cliffs are major geological landmarks and tourist sites.
Frequently asked questions
Is chalk a type of limestone?
Yes. Chalk is a soft, fine-grained, porous variety of limestone made mostly of microscopic marine plankton skeletons of calcium carbonate.
Is blackboard chalk made from real chalk?
Usually not anymore. Most modern classroom chalk is made from gypsum (calcium sulfate), though natural chalk was used historically.
Why is chalk so white?
It is composed almost entirely of pure calcium carbonate from plankton, with very little clay or iron, giving it a bright white color.
How can I tell chalk from other white rocks?
Chalk is very soft, leaves a powdery mark, and fizzes in acid. White clay and diatomite are also soft but do not react with acid.
Chalk guides
In-depth guides for identifying, valuing, and understanding Chalk.
