fluorite

Calcium fluoride, mined in Britain chiefly as 'fluorspar' — historically a discarded waste mineral in lead veins before becoming a valued industrial raw material, used as a flux in steelmaking and as the feedstock for hydrofluoric acid and the wider fluorochemical industry. The North Pennine orefield, especially Weardale, is Britain's classic source and produces prized gem-quality purple, green and blue crystal specimens alongside industrial-grade ore.