Calcium carbonate is made up of three elements: calcium, carbon and oxygen. This combination is a chemical compound, which is written out as CaCO3. It has one molecule of calcium, one of carbon and three of oxygen.
Calcium carbonate is widely used in calcium supplements or antacids; over consumption in these forms can cause illness. It is also found in rocks and is not geographically restricted to certain areas: calcium carbonate can be found all over the world. If you tested eggshells, you would also find a high concentration of the compound. One of the most common ways to find calcium carbonate is in the pearl jewelry that many people wear. Pearls and many other marine organisms have the compound as part of their makeup. Most marine organisms have calcium carbonate in their shells.