Chalk is made of the calcareous remains of zillions of microscopic fossil plankton (either coccolithophors or foraminifera). It is essentially a very fine-grained, uncompacted bioclastic limestone. The normal color is white, and yes, it was once used to make chalk for chalkboards. Today most chalkboard and sidewalk chalk is made from gypsum. Go figure.

The famous White Cliffs of Dover are made of chalk. These are the not-so-famous white cliffs of Yorkshire, located some distance north of Dover, England.
The white and grayish rock in this photo of the Dover cliffs is all chalk. Darker colors generally indicate more organic matter, but higher concentrations of clays can also darken chalk.
More chalk. It is quite blinding to work with on a sunny day.

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