Just want to say that bigbird, you are stupid for wasting money on a completely useless rock when you are living in such conditions. You have no right to be wasting that sort of money so I frankly don't feel bad about the price you paid because even spending a penny would have been too much considering your state of affairs.
Secondly, these types of things are easy to avoid. I've been going to Grand Cayman for the past ten years and buying jewelry. The key is to shop around. Impulse buying gets you no where. If you don't know the stone that you are buying, do some research first. Go around to a lot of different stores so you can come back and compare prices and try to pit two companies against each other. And most importantly, DON'T BE AFFRAID TO WALK OUT!!!! If they won't come down then leave. That rock is not that important that they can tell you what you will pay for it, you will tell them how much and if they won't come down, leave. Supply and Demand!
Be smart about your purchases, and I can promise right off the bat that anything you buy on a cruise is going to be overpriced and not the best quality, it's a cruise for goodness sakes. I would suggest going into town, or coming back to a place you had visited to buy at a later date. Waiting to make a big purchase after careful planning and consideration is key.