Solidus

Introduced during the reign of Constantine the Great, the Solidus (plural solidi) replaced the aureus as the standard gold coin of the Roman empire. Solidi were struck at a high purity of around 98% and weighed around 4.45 grams. Unlike the aureus whose weight and purity changed throughout its existence, the solidus would last for nearly 600 years relatively unchanged. Solidi were struck in huge numbers and were used to project the wealth and power of the Roman empire throughout the world. Today, solidi are relatively common as a denomination and are actively collected, although some issues, like that of Romulus Augustus, can command six figure prices.