Thanks to the mining and metallurgy of the prior worlds, and their ability to create anything they wished (or so it seems, anyway), no metal is rarer than any other.
People in Pangaea have no concept of gold, silver, gemstones, or even diamonds as being valuable due to their scarcity. Such materials are valuable based on their beauty or usefulness alone. Most civilized societies use generic coins commonly referred to as coppers.
Coppers are usually metal but can be made of glass, plastic, or substances that have no name. Some are jagged bits of interesting material or small, coinlike objects (such as highly decorative buttons from a machine), and others are properly minted and stamped, with writing and images. No minted coin in existence today comes from a prior civilisation — no coins survive from the ancient races if indeed they used such currency at all.
Some regions of Pangea accept only coins that were minted in that realm; others accept all coins, regardless of origin. This custom varies from place to place and society to society. Because coppers are from the current world, they rarely turn up in old locations. Occasionally, explorers of ancient or forgotten sites find a smattering of items—buttons or doodads—that can be salvaged as shins.
(unashamedly stolen from Numenera, which was unashamedly stolen from Planescape: Torment).