The Collaborative International Dictionary of English v.0.48:

Gerah \Ge"rah\, n. [Heb. g?rah, lit., a bean.] (Jewish Antiq.)
   A small coin and weight; 1-20th of a shekel.
   [1913 Webster]

   Note: The silver gerah is supposed to have been worth about
         three cents; the gold about fifty-four cents; the
         weight equivalent to about thirteen grains.
         [1913 Webster]

Easton's 1897 Bible Dictionary:

Gerah
   a bean, probably of the carob tree, the smallest weight, and
   also the smallest piece of money, among the Hebrews, equal to
   the twentieth part of a shekel (Ex. 30:13; Lev. 27:25; Num.
   3:47). This word came into use in the same way as our word
   "grain," from a grain of wheat.