Copper-impregnated limescale. Limescale is a mineral deposit formed inside kettles and plumbing. The limescale composition depends on whether it was formed from hot water (in which case it is calcium carbonate), or whether it was formed by the evaporation of 'hard' water (in which case it may also contain magnesium carbonate). Here, the mineral is blue in colour, due to the presence of hydrated copper ions. The copper is from the piping.