A term now used to designate any one of a family of
   minerals, hydrous silicates of alumina, with lime, soda, potash, or
   rarely baryta. Here are included natrolite, stilbite, analcime,
   chabazite, thomsonite, heulandite, and others. These species occur of
   secondary origin in the cavities of amygdaloid, basalt, and lava, also,
   less frequently, in granite and gneiss. So called because many of these
   species intumesce before the blowpipe.