Another vote for shopping in St. Maarten. Its is much less commercial, more laid back than St.Thomas, and the vendors are more willing to make a deal, we found the deals to be better on the Dutch side of the island. The key is to know your prices and the quality, cut,clarity and carat weight of a similar diamond back home. Attend the port shopping lectures. We found that the stores in St. Thomas were overcrowed, and the vendors wouldn't give you the time of day if you were just browsing. But they were all over you if you were going to drop a serious chunk of change in their store.
However, if you find something in St. Thomas that you totally love, and you feel that it is a good quality diamond and the price is right, buy it. IF you don't you will be dissapointed. Its not like you can just run back to the original store and buy the diamond that you passed over hoping to find a better one in St. Maarten.
HINT: St. Maarten - take the water taxi from the pier into downtown on the Dutch side its like $5.00 for an unlimited all day pass.