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]