The gerah, "a bean," weighed a little less than half of a dram avoirdupois (.439 dram).
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.