What is limestone made of?

Calcium carbonate, almost entirely. But that is the chemistry, not the story.

Pick up a piece of limestone and you are holding the floor of an old sea. Most limestone is built from shells, coral and the skeletons of sea creatures. They sank, piled up, and were pressed together over millions of years.

Some limestone formed differently, settling straight out of the water as a fine mud. Either way, it began underwater.

Worth knowing
Chalk is limestone too. It is made almost entirely from the plates of microscopic algae, each one far too small to see. The White Cliffs of Dover are billions of them. Same mineral, laid down a different way.

Source: British Geological Survey.