Calcium fluoride (CaF2) is an insoluble ionic compound of calcium and fluorine. It occurs naturally as the mineral fluorite (also called fluorspar), and it is the source of most of the world's fluorine. It reacts with concentrated sulfuric acid to produce hydrogen fluoride:
CaF2(s) + H2SO4(l) → CaSO4(s) + 2 HF(g)