Gold cannot actually rust; only oxidise. In order to rust, a compound would have to contain iron elements. Therefore, as stated in the FAQ for the update which released them, the coins in Daemonheim are not composed of gold, but rather of a material similar to novite (or novite itself).
The examine, which is "They make the world go round" is a reference to the common idiom used in the real world, "Money makes the world go round".
Along with the update on 16 January2012, they have changed to a bright orange colour, as the previous grey colour was not visible on some floors.
They were introduced to the game with the money pouch replacing the normal coins previously used in Daemonheim to make the difference between Daemonheim coins and normal coins more clear.