In the UK, hard water may contain the equivalent of 200 mg of calcium carbonate (limescale). Hard water also has a high mineral content, which can affect the colour of limescale.
When water is heated, dissolved calcium and magnesium bicarbonates break down and form solid limescale.
Limescale from hard water is usually white, but traces of other minerals or metals can change its colour. For example, copper traces in plumbing can make it grey, green or blue. Rust from water pipes can make it brown or black.