Apophyllite is a naturally occurring mineral group composed mainly of hydrated potassium calcium silicate fluoride. Its general chemical formula is KCa₄Si₈O₂₀(F,OH)·8H₂O. It belongs to the phyllosilicate mineral group and crystallizes in the tetragonal crystal system. Apophyllite is well known for forming beautiful, well-defined crystals, often in pyramidal or cubic shapes, with highly reflective surfaces. The mineral is typically colorless or white, but it can also appear in shades of green, pink, or yellow depending on trace elements. Transparent to translucent crystals with a strong vitreous (glass-like) luster are especially prized by collectors. Some specimens display a pearly sheen on cleavage surfaces, enhancing their visual appeal.