How do we get our alum blocks?

I had presumed that the alum blocks we use after shaving were mined mineral deposits just carved out of the rock and cut to size, much as common salt (sodium chloride) may be mined; but watching a TV programme on the North Yorkshire coast I now wonder if that is the case.

Is alum mined anywhere in a pure crystalline form, or is it always the product of a longer process which ends in the crystallization of a liqueur?