Double denarius coin. Roman emperor Caracalla (211-217 CE) introduced as a pure silver coin but soon debased to 3 percent silver. Depiction of emperor's portrait with a radiate crown is it's uniqueness.

A silver coin worth about 2 denarii

The name given in modern times to a Roman coin thought to be twice the value of a denarius. Originally a silver coin. The amount of silver in the mix dropped drastically over the years, and later antoniniani were hard to tell from copper or bronze.