The ‘Gerah’ weight is a type of ancient weight used in the Land of Israel during the Iron Age (the 12th century BCE-586 BCE). There is a disagreement among researchers whether the ‘Gerah’ constituted 1/20th of a ‘Shekel’ or 1/24th of a ‘Shekel’. Per the first view, the ‘Gera’ probably weighed on average about 0.569 grams. Per the second view, the ‘Gerah’ probably weighed on average about 0.47 grams.
‘Gerah’ weights were first discovered by Irish archaeologist Robert Macalister during his excavations at Tel Gezer. These miniscule weights were inscribed only with hieratic (one of the ancient Egyptian forms of writing) numerals. Later, many more such miniscule weights were discovered all over the Land of Israel, with different measurements and different hieratic numerals. It was hypothesized by some researchers that inscribing the name of the weight upon such a tiny object was a difficult task and would have also significantly eroded the weight, thus decreasing its value (unlike larger weights). It should be noted that a ‘1-gerah’ weight has yet to have been uncovered. So far, only weights representing multiple gerot (plural of gerah) have been found (such as 3 gerah, 5 gerah, etc).
The word ‘Gerah’ as the name of the miniscule weights within the ‘Shekel’ weight system was taken both from the Bible and from the similar weight system used in Babylon and Assyria, which included a weight called a ‘Giru’. In 2003 an ostracon from the Judah hill country was published. This ostracon described two monetary transactions and mentioned the Hebrew word ‘[G]RH’ ([Ge]rah) beside the icon the symbolizes the ‘Shekel’, which verified the ‘Gerah’ being part of the ‘Shekel’ system.
Sources:
G. Barkay, ‘Iron Age Gerah Weights (Pls. נה–נו)’, in: B. Mazar (ed.), Yohanan Aharoni Volume (Eretz-Israel: Archaeological, Historical and Geographical Studies 15), pp. 288-296
E. Eshel, ‘A Late Iron Age Ostracon Featuring the Term לצדכן’, IEJ 53 (2003), pp. 151-163
R. Kletter, Economic Keystones: The Weight System of the Kingdom of Judah, Sheffield 1998
R. Kletter, ‘’Four hundred shekels of silver, what is that between you and me?’ (Genesis 23:15): Weights and Weighing in Eretz Israel in Antiquity’, in: O. Ramon et al (eds.), Measuring and Weighing in Ancient Times (Catalogue 17), Haifa 2001, pp. 1-7 [Hebrew]
R. A. S. Macalister, The Excavation of Gezer: 1902-1905 and 1907-1909 – Volume II, London 1912