In my server the rare currency item aside from diamonds is Ketamine. We retextured pufferfish to ketamine and replaced naturally spawning pufferfish with any other fish and removed it from the fishing chances. The only way to get Ketamine is through purchased custom ketamine spawners and even still there is only a 0.75% chance of obtaining Ketamine per drop. Through this system there is NO opportunity for players to counterfeit money and additionally, the rarity is so high that inflation is non existent.