When people go to warmer climes for some R&R it is a natural to want a momento of the occasion - a ring or necklace to remember your time in the islands, etc. That is fine, but please let me offer some advice when it comes to making a jewelry purchase on vacation.
1. If the deal sounds too good - it is. A big, deep color "tanzanite" that does not cost several hundred dollars per carat probably isn't tanzanite at all. A heavy gold necklace, even if it is stamped "14KT" and "Italy" that you can buy for waaaay less than you expect is probably not gold at all. I have seen some very convincing fakes (got fooled once myself earlier this year!).
Remember - gold and diamonds are traded globally - they have a value no matter where on the planet you are. If gold is trading at $850 an ounce, then someone trying to sell you a 14kt gold necklace that weighs 20 grams for $150 is ripping you off unless he or she stole the necklace in the first place!
What CAN you do? First, if you get any kind of a hinky vibe from someone selling you a piece of jewelry, then walk away. Look for established businesses - don't buy a "gold" watch from some guy on the beach.
Secondly, if you are making what you consider to be a major purchase, then put it on your credit card and have the item examined by someone you trust when you get back home. That way you can dispute the charges if your 1 carat diamond turns out to be a CZ or white sapphire when you get home.
Make your buying decision when you are sober! Many places are happy to serve you a couple drinks to make the decision making "easier" for you.
Examine what you are buying - look for quality stamps (ask to borrow their loop or a magnifier) and ask for a written receipt that states what the jewelry is supposed to be.
Have fun and get some sun - just use your head when you buy some bling.