The main components of these minerals usually are calcium (Ca) and magnesium (Mg) ions. But there are also dissolved metals, bicarbonates, and sulfates. Calcium usually enters the water as either calcium carbonate (CaCO) in the form of limestone and chalk, or calcium sulfate (CaSO) in the form of several other mineral deposits. The main source of magnesium is dolomite (CaMg(CO)). Hard water is generally not harmful and some hardness in water is a useful source of calcium for healthy bone development . 