Found this through a quick google - "Water becomes hard by dissolving soluble salts from the rocks or soil over or through which it flows. Some rocks, for example slate, granite, gneiss and schist, contain little or no soluble material and, as such, have a negligible effect." - then there was one site that suggested to stay away from it, that's the first time I've ever heard that, in contrast to the norm, so it goes on the list of many web pages I'll ignore from now on.
Quickest way to tell is scrape the surface and drip some hydrochloric or acetic acid from a liquid nitrate or nitrite test kit and see if it sizzles.
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