Hard water is caused by the chalk/limestone (Calcium Carbonate) deposits present in the ground dissolving into the rain water which when it has been treated eventually comes into your house.
Hard Water is effectively a solution of Calcium Carbonate (CaCO3). When hard water is heated up in your house or when the water pressure changes as it does when leaving a tap or suddenly changing direction in an L-bend pipe, the Calcium Carbonate comes out of solution and reverts back to chalk once again. This chalk is of course now called limescale but is still hard deposits of chalk.
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Here are some images of Limescale and places you are likely to find them:-