You always hear about someone buying a piece of jewelery on some island, getting back home and taking it right away to Fred the friendly neighborhood jeweler and HE said --it's was worth much more than they paid, etc. etc.
First of all, you usually do not get something that's actually worth $ 4000.
for $ 1500.
If they trust Fred the friendly neighborhood jeweler to tell them what it's worth, why not trust him enough to buy from him in the first place ?
I would wager that most of these jewelers back home won't put in writing what they tell you re/ your purchase.
My wife has been in the jewelry business, managing a store for one of the largest jewelers in the country ( approx. 1200 stores nationwide ) for well over 30 years.
They don't do appraisals as it was beginning to be a problem when they told people what the item was worth, instead of telling them what a great deal they got.
I'm not at all saying you can't get a good deal on some of the islands--I'm repeating what has already been said--be very careful and know your stuff and your prices at home prior to plopping down a bunch of money. The person you are buying from isn't giving you a great deal because they like you--they may come down in price but they have to make a profit or go out of business.
The only people my wife ever tells she's in the jewelry business when we cruise are the people who try to entice you into their stores--when they find out she's in the business, they will immediately back off and wait for the next " customer " to come by.
Remember-- tourists and their money, in a lot of cases, can easily be separated.