as you play through the game you will come across treasures you can sell to the duke
some of these treasures can be kept until a later time when you have a few parts that combine to make a better treasure that sells for a much higher price than the combined value of the pieces. I'll write a more detailed guide in the future for every treasure, as after finishing the game I found i missed making 2 of them.
when looking to see which treasure to keep and what to sell. check your descriptions and if they say (combinable) then it's needed for a larger treasure. if it ONLY says valuable or highly valuable or something like that and there is nothing saying Combinable, then it is safe to sell when ever you wish