During the game we can gather stuff which are needless and nobody wants to buy them... After some there is a huge amount of these things it might have been an additional option of selling them like minerals. In exchange for crowns of course, for example: 0 stars - 1 crown, 1 star - 2 cr, 2 stars - 3cr, 3 stars - 4 cr, 4 stars - 5 cr, 5 stars - 6 cr...