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Hard water

It's chalk (calcium carbonate) dissolved in the water. Its origin is geological presumably because the water dissolved mineral from rocks. It ends up in kettles, water tanks, etc. See kettle descaler and Scale Watcher. How hard water interacts with TEA and what can be done about it, that's a subject which is a speciality of Yorkshire Tea

Limescale from hard water can reduce the efficiency of some machines, but not electric heating elements. More about this at the page of Limescale and the efficiency of electric water heaters