"Medium of Exchange" and "Medium of Account"

Two items from The Coinage of Ancient Rome:

During which emperor's reign was nearly all silver removed from Roman coins?

Gallienus. When Gallienus (253 - 268 a.d.) became Emperor, the coinage was already very debased. During his disastrous reign almost all silver was removed. He tried to disguise this fact by issuing copper coins which were silver-plated.

The standard gold coin of the early empire was the "aureus." How much gold did it contain?

About 1/5 oz. Unlike the silver coinage, Roman gold coins continued to be minted from good metal through the history of the Empire, undergoing only minor debasement. The weight of the aureus did tend to fluctuate a bit, but it was usually minted at 60 aureii to the Roman pound, which works out to about 1/5 oz of precious metal.

When wealth is excessively concentrated, little or nothing remains to serve as value in the everyday money of everyday folk. But the gold coins, that's a different matter.