The aureus (pl. aurei — "golden") was a gold coin of ancient Rome valued at 25 silver denarii. The aureus was regularly issued from the 1st century BC to the beginning of the 4th century AD, when it was replaced by the solidus. The aureus is about the same size as the denarius, but is heavier due to the higher density of gold.
Before the time of Julius Caesar the aureus was struck very infrequently, usually to make large payments from captured booty. Caesar struck the coin more frequently and standardized the weight at of a Roman pound (about 8 grams). Augustus tariffed the value of the sestertius as 1/100 Aureus. The mass of the aureus was decreased to of a pound (7.3 g) during the reign of Nero.
After the reign of Marcus Aurelius the production of aurei decreased, and the weight was further decreased to of a pound (6.5 g) by the time of Caracalla. During the 3rd century, gold pieces were introduced in a variety of fractions and multiples, making it hard to determine the intended denomination of a gold coin.
The solidus was first introduced by Diocletian around 301 AD, struck at 60 to the Roman pound of pure gold (and thus weighing about 5.5 g each) and with an initial value equal to 1,000 denarii. However, Diocletian's solidus was struck only in small quantities, and thus had only minimal economic effect. The solidus was reintroduced by Constantine I in 312 AD, permanently replacing the aureus as the gold coin of the Roman Empire. The solidus was struck at a rate of 72 to a Roman pound of pure gold, each coin weighing twenty-four Greco-Roman carats, or about 4.5 grams of gold per coin. By this time, the solidus was worth 275,000 increasingly debased denarii.
However, regardless of the size or weight of the aureus, the coin's purity was little affected. Analysis of the Roman aureus shows the purity level usually to have been near to 24 carat gold in excess of 99%, 23 carat of 95.83% gold English Sovereign (1489–1604), 91.7% of 22 carat gold for the British Sovereign (1817–1917, 1925, 1957–present) and American Eagle (1795–1933), and 21.6 carat of 90% gold for the United States Gold Dollar (1849–1889).
Due to runaway inflation caused by the Roman government issuing base-metal coinage but refusing to accept anything other than silver or gold for tax payments, the value of the gold aureus in relation to denarii grew drastically. Inflation was also affected by the systematic debasement of the silver denarius which by the mid-3rd century had practically no silver left in it.
In 301, one gold aureus was worth 833⅓ denarii; by 324, the same aureus was worth 4,350 denarii. In 337, after Constantine converted to the solidus, one solidus was worth 275,000 denarii and finally, by 356, one solidus was worth 4,600,000 denarii.
Constantine introduced the solidus in 309, replacing the aureus as the standard gold coin of the Roman Empire. The solidus was a larger diameter and flatter coin, while the aureus was smaller, thicker and similar to the denarius in fabric. Today, the aureus is highly sought after by collectors because of its purity and value, as well its historical interest. An aureus is usually much more expensive than a denarius issued by the same emperor. For instance, in one auction, an aureus of Trajan sold for $15,000, and a silver coin of the same emperor sold for $100. Two of the most expensive aurei were sold in the same auction in 2008. One aureus, issued in 42 BC by Marcus Junius Brutus, the assassin of Gaius Julius Caesar, had a price realized of $661,250. (There is an example of this coin on permanent display at the British Museum in London.) The second aureus, issued by the emperor Alexander Severus, has a picture of the Colosseum on the reverse, and had a price realized of $920,000.