Doubloons
They are used to make Sigils of Luck and Runes of the Pirate.
http://wiki.guildwars2.com/wiki/Superior_Sigil_of_Luck
http://wiki.guildwars2.com/wiki/Rune_of_the_Pirate
From the fact that leatherworker can also use it, I believe there may be a third recipe, but I’m not sure what it is.