"I will give man his threescore and ten, and then give him more. Death himself will fall before me."
Orcish currency is derived from glass beads. The art of glassworking is well beyond them, but perhaps the orcs have something of value to the civilized races, such as animal pelts, and well made axes, and bows. The humans trade beads for the goods, and the orcs will trade the beads amongst themselves as a form of their own currency. Perhaps they value blood red beads above all others, or animistic orcs favor beads in the colors of their gods.
Inspired by Indian trade beads, some of which could be quite ornate and beautiful. Most North American Indian beads were made in Italy. Surprise!