Chalk is a white crumbly rock made of microscopic shells and is rich in calcium carbonate. Chalk is softer than limestone and is used to make a substance called putty. Putty is used to fill holes in woodwork, give a smooth finishing to the walls of buildings and seal glass into windows. Chalk is also ground into a powder called whiting, which is used in crayons, rubber paints and cosmetics.
Limestone often contains the fossils of dead animals and plants that were trapped in between the layers of sediment. Limestone is hard enough to be qualified as a building material; it is used to construct buildings, walls and paving stones. Sometimes, limestone contains phosphates. This is used to make fertilizers.
Marble is formed when chalk or limestone is buried deep underground. The pressure and heat changes the limestone or chalk into marble. It is hard, yet shiny and beautiful. Therefore, marble is used in flooring. Many sculptures and buildings around the world are made of marble. The famous Taj Mahal is built entirely of marble.
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