Have discussed this with an early 20s family member, to see what they can come up with. Their conclusions:
Very tentative possibility is the Sun Sword Trilogy by Belinda Murrell, but that doesn't really fit with the idea of a single book and it doesn't appear to have been released as an omnibus. There are ocean battles, but with people who are better described as privateers than pirates.
Another long shot would be a Deltora Quest Omnibus -- in the first 8 books they are after gems, in the next 3 they are after magic items, and in the last four they are hunting and destroying magic items. But no pirates. Not sure exactly which books might have been collected together, but there is definitely a collection of 1-4
But other than that, nothing really comes to mind. Gems apparently turn up a bit, but not in conjunction with the other details.
no subject
Very tentative possibility is the Sun Sword Trilogy by Belinda Murrell, but that doesn't really fit with the idea of a single book and it doesn't appear to have been released as an omnibus. There are ocean battles, but with people who are better described as privateers than pirates.
Another long shot would be a Deltora Quest Omnibus -- in the first 8 books they are after gems, in the next 3 they are after magic items, and in the last four they are hunting and destroying magic items. But no pirates. Not sure exactly which books might have been collected together, but there is definitely a collection of 1-4
But other than that, nothing really comes to mind. Gems apparently turn up a bit, but not in conjunction with the other details.