During the game, starting from Reaper's Coast onwards, there are various single pieces of gold available for stealing / pick up in the environment.
These gold pieces all have a default value of 0 (rather than 1) and although they stack with each other, they don't stack with the main gold stack. They are also not used as currency for balance offer.
They can of course be sold for 1 gold each (min value I suppose) to traders, but it looks like an oversight rather than intended that they don't count as actual currency
