Really am sorry that you got "stuck".
But, as a generality, not sure why anyone would think to buy expensive loose stones in Cozumel... from any store.
When spending those kinds of dollars on stones ANYWHERE you need to know your stuff, and know values. You can't simply walk into a store and say.. that looks like a nice one.
Cruise line "recommended stores" are a joke. They pay the cruise lines to be recommended.