The Roman coinage system was complex and transformed over time. The denominations we use are asses, denarii, antoninianii, and folles.
Roman coins made of gold and silver were roughly worth their weight. The as, however, was a bronze coin, a material which is worth almost nothing in such a small quantity. Instead, these coins were worth what the government declared they were worth, the same way our money works today.
The Denarius and the Antoninianus
The denarius was the silver coin of the 1st and second centuries. Over time, cash strapped emperors replaced the silver with increasing amounts of bronze. In the third century, the denarius was replaced by a larger silver coin Romans described as a double denarius and which we call an antoninianus. It was not, incidentally, double the weight of denarii silver. Over time, this coin was debased as well. First, silver and bronze were mixed. Eventually, the coin was entirely bronze with a silver wash across the surface.
A hundred later, another coinage reform replaced the antoninianus with the follis, another bronze coin with a silver wash. Over the next hundred years, the follis varied in size, from about 22mm to 14mm. The silver wash on both types of coins has largely worn away, leaving no trace. Every follis we use was once covered in silver.