An extremely valuable ghostly coin.
How to get
Trading with the Ghost Ship from Ghost Ship Cove at the Docks.
Blowing up the cave at Ghost Ship Cove
with a Powder Keg
You may also receive a Doubloon when opening an Eternal Ghost Chest .
Unlike most other currencies Ghost Doubloons can sometimes be found for sale in the World Market.
There are occasionally "Double Doubloon" events during which the Ghost Ship Captain trades two doubloons per order rather than one, and the boat comes to the dock more frequently.
Ghost Doubloons are used to buy special items from the trade panels at Ghost Ship Cove. See here Selling and here Schedule for the 3rd Trade Panel to see what is available!