Iron and manganese are often naturally occurring and are predominantly found in groundwater. We have a lot of it here in New Hampshire. Iron can lead to reduced water pressure by building up inside of your pipes. Manganese often makes the back of the toilet tank black.
Water is a natural solvent, and given the needed time and conditions, it will dissolve anything it comes in contact with. That is why, depending on where you live, your water can contain iron and manganese, which can cause rusty-orange or black staining on water-using appliances, fixtures, tubs, sinks, toilets and clothing.