This would probably depend on woods getting different values, but it think it would be cool if there was the option for you to make glass or wooden coins in fortress mode, and allow entities to use glass or wooden coins.
Reason? Well, elves use metals for their coins. Seeing as they have no metal themselves, they must have traded for metals for their coins and hired some dwarf or human to mint the coins for them.
Which is really odd if the elf civilization is completely cut off from the other civs. So they should have an option to make coins out of the materials they have. It also seems pretty elfy to me.
Maybe to add value to the wood you could have various kinds of lacquer which can be made from some natural plant/tree products, which they can then treat the wood with. Which would make lacquered feathertree wood for instance. It should be a material type, so no masterfully lacquered wood of any type, kind of like smelting. It could also presumably add strength to wooden weapons and armor. Lacquering should be unique to elves (and modded races of course), since no one else really needs it.
And the reason for glass coins? Cuz it would just be awesome for a modded in desert volcano civ (viable with editor in next release) to have green glass 'copper coins', clear glass 'silver coins' and crystal glass 'gold coins'.
Stone coins would also be viable i guess, i'm pretty sure some ancient civilization used some sort of stone coins.
Imagine an evil race with obsidian coins with an image of a dragon on one side and a demon surrounded by orcs (or something) on the other side. The orcs are bowing to the demon.
Edit: Heck, even ash/potash/pearlash coins would be fun. Soap or bone isn't really viable because it depends on what creatures it is made of. Imagine a civ that decides all it's coins have to be made of dragon, hydra and titan bones. Those coins would be freakin' rare. And still only worth 5 credits.
And gems are just too rare.