The aureus (pl. aurei) 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 approximately 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 1/40 of a Roman pound (about 8 grams). The mass of the aureus was decreased to 1/45 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 1/50th of a pound (6.5 g) by the time of Caracalla. During the third century, gold pieces were introduced in a variety of fractions and multiples, making it hard to determine the intended denomination of a gold coin.
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 in excess of 99%, compared to 91.7%(22k) for the British sovereign and the 90% for the US gold dollar.
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-third century had practically no silver left in it.
In 301 AD one gold aureus was worth 8331/3 denarii; by 324 AD the same aureus was worth 4,350 denarii. In 337 AD, after Constantine converted to the solidus, one solidus was worth 275,000 denarii and finally, by 356 AD, 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 and chunky and similar to the denarius in fabric.