What causes hard water?

Hardness minerals – calcium and magnesium – are in plentiful supply. While they are not found in their elemental form in the earth, they occur in combination with other elements in an abundance of forms. Common calcium minerals include chalk, limestone, and marble. These substances are chiefly calcium carbonate (CaCO3) or mixtures of calcium and magnesium carbonates and other impurities. The hardness in water that is caused by calcium, magnesium, and other cations is usually described in terms of the calcium carbonate equivalent.