A siliqua (pl. siliquas or siliquae) is a small, thin, Roman silver coin produced starting in the 4th century.
The name siliqua is modern; its ancient name is unknown. The identification with the siliqua auri or keration was proposed by Theodor Mommsen in 1860, but this identification is now rejected.
The siliqua was a unit of weight equal to one twenty-fourth of a solidus. The word comes from the siliqua graeca, the seed of the carob tree (cf. carat), which in the Roman weight system is equivalent to 1⁄6 of a scruple (1⁄1728 of a Roman pound or about 0.19 grams).
The term has been applied since the 19th century to various silver coins on the premise that the coins were valued at 1⁄24 of the gold solidus (which weighed 1⁄72 of a Roman pound) and therefore represented a siliqua of gold in value. Since gold was worth about 12 times as much as silver in ancient Rome (in Diocletian's Edict on Maximum Prices of 301), such a silver coin would have a theoretical weight of 2.22 grams (12 × 4.45 g/24 = 2.22 g). The term is more generally applied today to silver coins issued by Constantine, which initially weighed 3.4 grams, and to the later "heavy siliqua" of Constantius II of c. 3 grams, but it would fit the later "light" or "reduced siliqua" from after the reform of 355 which weighed about 2.2 grams. Thin silver coins as late as the 7th century which weigh about 2–3 grams are known as siliquas by numismatic convention.
Most examples suffer striking cracks (from rapid production) or extensive clipping (removal of silver from the edges), so untouched and undamaged examples are uncommon.
