What Is Chalk?
Chalk is a soft, white limestone. The chemical composition is calcium carbonate with minor amounts of silt and clay. It was formed from the shells that belonged mostly to tiny sea animals called foraminifera. These tiny creatures have lived on the earth for millions of years.
In some places long ago, they lived in such numbers that their shells made a layer hundreds of feet thick on the floor of the sea. The layer of shells gradually became a layer of white rock which we call chalk. Long ago, people found that chalk was so soft; it could be used to write with. For many years, almost all blackboard crayons were made of pure chalk.
Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulfate), often supplied in sticks of compressed powder about 10 cm (4 in) long.
Sidewalk chalk is similar to blackboard chalk, but it is shaped into larger sticks and often colored. It is used to draw on sidewalks, streets, and driveways. Tailor’s chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. However, it is now usually made from talc (magnesium silicate).