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.
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.
The antoninianus (actual name unknown) was a coin used during the Roman Empire that was valued at 2 denarii. It was initially silver, but was slowly debased to bronze.