Solidus (coin)

The solidus (Latin 'solid'; pl.: solidi) or nomisma (Greek: νόμισμα, romanized: nómisma, lit. 'coin') was a highly pure gold coin issued in the Later Roman Empire and Byzantine Empire. It was introduced in the early 4th century, replacing the aureus, and its weight of about 4.45 grams remained relatively constant for seven centuries.

In the Byzantine Empire, the solidus or nomisma remained a highly pure gold coin until the 11th century, when several Byzantine emperors began to strike the coin with less and less gold. The nomisma was finally abolished by Alexios I Komnenos in 1092, who replaced it with the hyperpyron, which also came to be known as a "bezant". The Byzantine solidus also inspired the zolotnik in the Kievan Rus' and the originally slightly less pure gold dinar first issued by the Umayyad Caliphate beginning in 697.

In Western Europe, the solidus was the main gold coin of commerce from late Roman times to the Early Middle Ages.

In Late Antiquity and the Middle Ages, the solidus also functioned as a unit of weight equal to 1⁄72 Roman pound (approximately 4.45 grams).

The solidus was initially introduced by Diocletian in small issues and later reintroduced for mass circulation by Constantine the Great in c. AD 312 and was composed of relatively solid gold. Constantine's solidus was struck at a rate of 72 to a Roman pound (of about 326.6 g) of gold; each coin weighed 24 Greco-Roman carats (189 mg each), or about 4.5 grams of gold per coin. By this time, the solidus was worth 275,000 increasingly debased denarii, each denarius containing just 5% (or one twentieth) of the amount of silver it had three and a half centuries beforehand. With the exception of the early issues of Constantine the Great and the odd usurpers, the solidus today is a much more affordable gold Roman coin to collect, compared to the older aureus, as well as those of Heraclius, Honorius and later Byzantine issues.

The solidus was maintained essentially unaltered in weight, dimensions and purity, until the 10th century. During the 6th and 7th centuries "lightweight" solidi of 20, 22 or 23 siliquae (one siliqua was 1/24 of a solidus) were struck along with the standard weight issues, presumably for trade purposes or to pay tribute. The lightweight solidi were especially popular in the West, and many of these lightweight coins have been found in Europe, Russia and Georgia. The lightweight solidi were distinguished by different markings on the coin, usually in the exergue for the 20 and 22 siliquae coins, and by stars in the field for the 23 siliquae coins.

Despite the Eastern half of the Roman Empire being predominantly Greek speaking, its coins were still inscribed in Latin well into the eighth century. The letters in the inscriptions began to lose their Classical Latin look under the emperor Heraclius, and the Latin text was replaced with Greek script in the early years of the ninth century, during the reign of Constantine VI.

In theory the solidus was struck from pure gold, but the limits of refining techniques meant that, in practice, the coins were often about 23k fine (95.8% gold). In the Greek-speaking world during the Roman period, and then in the Byzantine economy, the solidus was known as the νόμισμα (nomisma, plural nomismata). In the 10th century Emperor Nicephorus II Phocas (963–969) introduced a new lightweight gold coin called the tetarteron nomisma that circulated alongside the solidus, and from that time the solidus (nomisma) became known as the ἱστάμενον νόμισμα (histamenon nomisma), in the Greek speaking world. Initially it was difficult to distinguish the two coins, as they had the same design, dimensions and purity, and there were no marks of value to distinguish the denominations. The only difference was the weight. The tetarteron nomisma was a lighter coin, about 4.05 grams, reminiscent of the lightweight solidi of the 6th and 7th centuries, but the histamenon nomisma maintained the traditional weight of 4.5 grams. To eliminate confusion between the two, from the reign of Basil II (975–1025) the solidus (histamenon nomisma) was struck as a thinner coin with a larger diameter but with the same weight and purity as before. From the middle of the 11th century, the larger diameter histamenon nomisma was struck on a concave (cup-shaped) flan, while the smaller tetarteron nomisma continued to be struck on a smaller flat flan.