A mineral belonging to the zeolite family of silicates. It crystallises in the monoclinic system in crystals that are tabular parallel to the side pinacoid. Most characteristic are sheaflike aggregates of thin tabular crystals. There is perfect cleavage parallel to the side pinacoid, and here the mineral has a pearly luster; elsewhere the lustre is vitreous. The colour is usually white but may be brown, red, or yellow. Hardness is 3½–4 on Mohs scale; specific gravity is 2.1–2.2. Stilbite is a calcium-sodium aluminium silicate, Ca(Al₂Si₇O₁₈) · 7H₂O, but usually contains some sodium replacing calcium.