Apophyllite (ah-POFF-fill-ite) is a potassium-calcium silicate fluoride hydroxide hydrate with the formula KCa₄Si₈O₂₀(F,OH).8H₂O. The name is from the Greek apos ("off") and phyllon ("leaf") because it exfoliates when heated before the blowpipe.

It is a mineral in the zeolite family and is very rarely cut, usually only for collectors. Apophyllite ranges in color from colorless to yellow, green, violet and pink, but it is usually violet or pink and semi-transparent in its gem quality; very rarely does it occur in transparent crystals.

Apophyllite often shows anomalous optic characteristics. Specific gravity and cleavage will separate it from chalcedony and quartz, and optic character from feldspar. It commonly grows in pseudocubic crystals, and on the basal pinacoids (the directions of perfect cleavage), it shows a characteristic pearly luster.

Apophyllite is found with zeolites in cavities in basaltic rock and also in granite and gneiss. Principal sources are Guanajuato, Mexico; the Bay of Fundy region, and Maine in the United States; the Rossland area of British Columbia, Canada. It also occurs in northern New Jersey, Pennsylvania, Virginia and Oregon in the United States, as well as Brazil, Sweden, Finland, Germany, Czechoslovakia, Iceland and Scotland. An old source is located near Poona, India.

Apophyllite is brittle and weak; therefore, it should only be ground on a 220 or 400 grit wheel and never into the grain of the cleavage. It should be polished on leather with Linde A or on felt with cerium oxide. Faceting material should be preformed and then cut on a diamond/copper lap, then polished on tin with Linde A. Crown angles should be 42° and pavilion angles 43°.