Celestite is a mineral form of Strontium Sulfate which is a metal.  The metal is mixed with Carbon and Sulfur which is used for the red “fire” in fireworks, flares and road signals.  Celestite occurs as Crystals and in compact massive and fibrous forms.  It is mostly found in sedimentary rocks and is often associated with Gypsum, Anhydrite and Halite.  Pale blue Crystal specimens are found in Madagascar.