Obolus is so yesterday!
This article refers to Inferno Ascended.
The currency used in the entire Stygian empire is based on gold and silver coins. Money is issued by the State and usually has the seal of origin. No matter the origin, the money is accepted all over the empire.
For lesser valuable items, small black stone tablets are used as currency. They are called Fish, Fowl, and Cattle, and usually have drawings of these animals to have illiterate folk to identify their value.
An old custom to put funerary coins on the mouth of the dead to pay for their good riddance to the Underworld, is still a practice. The coin specially made for it is called obolus.