Aureus

The aureus was the standard gold coin of ancient Rome. Gold was struck only occasionally under the Republic, but from the time of Julius Caesar the aureus was issued regularly and became the highest-value coin in common Roman use, worth 25 silver denarii. It remained in production until the early fourth century CE, when Constantine I replaced it with the lighter solidus.

The name comes from the Latin aureus, "golden", short for nummus aureus, "gold coin". Because of its high value it was used mostly for large payments, such as bonuses to soldiers, salaries of senior officials and major purchases, and as a store of value, rather than for everyday shopping.

Gold coins had circulated in the Mediterranean world since the first coins of Lydia, including the gold Lydian stater, and later the gold Macedonian stater of Philip II. Rome came to gold late. Its earliest gold coins were emergency issues struck during the Second Punic War, at about the same time as the first denarii, and for more than a century afterwards gold was rarely minted at Rome. Gold returned in the first century BCE in the hands of military commanders, including Sulla and Pompey, who struck coins to pay their armies. Julius Caesar's large issues, beginning around 46 BCE, made the aureus a regular part of the currency, at about 8 grams, or one fortieth of a Roman pound.

Augustus established a stable structure in which one aureus equalled 25 denarii, 100 sestertii or 400 asses. The coin carried the portrait of the emperor or members of his family, with reverse designs celebrating victories, virtues and building projects, so it spread official messages across the empire.

Unlike the silver coinage, the aureus generally stayed close to pure gold, and its value rested on its intrinsic value. What changed was its weight. Nero reduced it to one forty-fifth of a pound in 64 CE, about 7.3 grams, and Caracalla cut it again to about one fiftieth of a pound. These reductions allowed the state to strike more coins from the same gold, a form of currency debasement even though the fineness stayed high.

During the political and military crisis of the third century CE, the weight of the aureus became erratic, with coins of noticeably different weights struck in the same reign. The link between gold and the badly debased silver coinage broke down, and gold increasingly had to be valued by weight rather than by count. This illustrates a familiar problem of bimetallism: when the official ratio between two metals no longer matches their market value, people stop treating the coins as fixed equivalents.

Diocletian tried to restore order in the 280s and 290s by fixing the aureus at sixty to the pound, about 5.4 grams, and some coins were marked with the Greek numeral for sixty. His wider reforms also introduced new silver and bronze denominations.

Around 309–310 CE Constantine I introduced the solidus, struck at 72 to the Roman pound, or about 4.5 grams. It was first issued in the territories he controlled and spread across the whole empire after he defeated his rival Licinius in 324. The aureus went out of production, and the solidus became the gold coin of the later Roman and Byzantine world, known in medieval western Europe as the bezant.

Aurei have been found far beyond the imperial frontiers, including in India, which Roman gold reached through trade. The gold coins of the Kushan Empire were struck to a weight close to that of the aureus, and many scholars see a connection between the two. Aurei survive in far smaller numbers than silver coins, because gold was frequently melted down and recoined, and they are among the most valuable ancient coins on the collector market.

• British Museum: Collection online
• American Numismatic Society