I would definitely recommend Splash on Front Street in St. Maarten. Friends have bought from them for years and DH bought me a beautiful diamond ring this past summer! They are family run and I can't say enough good things about them. Honest, fair and excellent quality. They don't advertise much, because they get a lot of repeat business and word of mouth.
They used to be way, way, way down the end of Front Street - one of the last shops, so the only way they could stay in business is if people knew to look for them or were return shoppers. (I say that because any jewelry store in the middle of Front Street gets TONS of pedestrian traffic!) We got a postcard from them a few weeks ago that they had moved up to the center of the main street - near the courthouse.