Calcium carbonate is a white chemical compound found abundantly in nature. It's the main ingredient in limestone, marble, chalk, and seashells. When you pick up a piece of chalk or examine a seashell closely, you're holding calcium carbonate.

This compound forms when calcium (a common element) combines with carbon and oxygen. The white cliffs of Dover in England? Calcium carbonate. The beautiful marble columns in ancient Greek temples? Also calcium carbonate, transformed by heat and pressure deep underground. Even the shells that protect snails, clams, and oysters are built from it.

Calcium carbonate matters beyond rocks and shells. Your bones and teeth contain it, giving them strength. Farmers spread it on fields to reduce soil acidity. Manufacturers use it to make cement, paper, plastics, and even some medicines. Those chalky antacid tablets people take for heartburn? They often contain calcium carbonate to neutralize stomach acid.

In chemistry class, you might see calcium carbonate fizz when vinegar (an acid) hits it, releasing carbon dioxide bubbles. This same reaction slowly carved out massive underground caves over millions of years as slightly acidic rainwater dissolved limestone. Stalactites hanging from cave ceilings formed as that dissolved calcium carbonate dripped down and hardened again, one drop at a time.