That's it? No gems... No jewels?!- Aika's reaction after finding "Daccat's treasure" A single coin and a letter expressing that the most valuable treasure is friendship left by the Great Air Pirate Daccat.
The coin itself can be sold for 20,000 gold at the information center in Nasrad after you return from the Grand Fortress. This is very helpful when 100,000 gold is needed to pay Izmael and Brabham to advance in the story.