From BiblePortal Wikipedia

Hastings' Dictionary of the Bible [1]

Gerah , the twentieth part of the shekel (  Exodus 30:13 ,   Leviticus 27:25 etc.). See Money, 3  ; Weights And Measures, iii.

Easton's Bible Dictionary [2]

 Exodus 30:13 Leviticus 27:25 Numbers 3:47

American Tract Society Bible Dictionary [3]

The smallest weight or coin among the Jews, the twentieth part of a shekel, and worth about two and a half cents,  Exodus 30:13 .

Holman Bible Dictionary [4]

ShekelWeights And Measures

Morrish Bible Dictionary [5]

See Weights And Measures

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [6]

( גֵּרָה , Gerah', a Berry or granule [compare English "barley-corn" and "grain" as measure and weight]; Sept. Ὄβολος , Vulgate Obolus), the smallest weight, and likewise the smallest piece of money among the Hebrews, equivalent to the twentieth part of a shekel ( Exodus 30:13;  Leviticus 27:25;  Numbers 3:47;  Numbers 18:16;  Ezekiel 45:12). It would therefore weigh 13.5 Paris grains, and be worth about 3 cents. The same Hebrew word also signifies Cud, as being a round mass. It has been supposed by many that the gerah was so called from the fact that some kernel, as of pepper or barley, or perhaps the seeds of the carobtree ( Κεράτιον ) may have been originally used for this weight, but it would be equal in weight to 4 or 5 beans of the carob, and, according to the Rabbins, it weighed as much as 16 grains of barley. (See Metrology).

International Standard Bible Encyclopedia [7]

gē´ra ( גּרה , gērāh , "grain" or "kernel"): A weight, the 20th part of a shekel (  Exodus 30:13;  Leviticus 27:25;  Numbers 3:47;  Numbers 18:16;  Ezekiel 45:12 ). See Weights And Measures .

Kitto's Popular Cyclopedia of Biblial Literature [8]

Ge´rah, the smallest piece of money among the Hebrews. Twenty made a shekel; one of them would therefore be worth three halfpence, according to the present value of silver .