The aureus was a Gold coin of high value equivalent to 25 silver denarii. The Aureus was minted from the 1st Century BC to the beginning of the 4th century AD. It was eventually replaced with the solidus by Constantine.
The aureus is a similar size to the denarius but heavier due to its gold content.
The size of the aureus declined over time but remained almost pure cold.
Runaway inflation increased the value of the aureus dramatically until it was worth over 4000 denarii by the time it was converted to the solidus. In 356 AD, one solidus was worth 4,600,000 denarii.