The Collaborative International Dictionary of English v.0.48:

Prehnite \Prehn"ite\, n. [So called from the German Colonel
   Prehn, who first found it.] (Min.)
   A pale green mineral occurring in crystalline aggregates
   having a botryoidal or mammillary structure, and rarely in
   distinct crystals. It is a hydrous silicate of alumina and
   lime.
   [1913 Webster]