"If you can't prove you owned it before ...." Are you supposed to bring receipts for every piece of jewelry you take??? C'mon, now! I can guarantee that the vast majority of items purchased on cruises gets by customs with no problem. Not that I advocate such devious behavior, but it's done all the time. Dogs aren't trained to sniff out diamonds or emeralds; and wearing a nice ring or necklace doesn't automatically bring suspicion upon people. But to avoid being conspicuous, folks usually bury the expensive purchases in their suitcases. Since there are two of us traveling, we have no problem dividing the merchandise, so that our purchases fall within the allowable limits. While I'm not encouraging anyone to be dishonest, it's a fact of cruising life that most jewelry reaches its ultimate destination undeclared.
Speaking of deception and dishonesty, there's plenty of that all over the Caribbean. Many stones are "laser-enhanced" (i.e. treated to make bad stuff look good), "lab-created" (i.e. man-made) and otherwise helped along (e.g. soaking emeralds in oil to hide the imperfections). Technology has advanced so much, this trickery often fools experts. If you purchase one of these clunkers, you're out of luck, as your credit card company will not place items purchased further than 100 miles from your home state in dispute. If you're shopping in St. Maarten, Caribbean Gems and Joe's Jewelry (same company) are beyond reproach. There's some wiggle room with the prices; but both shops are very reliable. I personally don't buy anything from Colombia (including coffee!) since most of the money ends up in the hands of the cocaine cartels (I'm in law enforcement; trust me on this one), so I avoid purchasing emeralds. I also stay away from the "recommended" merchants. They pay thousands in kickbacks to the cruise lines, and you ultimately foot the bill. Merchandise is as represented, but you can do better. If, however, you need the comfort of the guarantee, you won't be hosed - you'll just pay a bit more. If you have a bit of will power, you won't pay the ticketed prices; so sharpen your negotiating skills.
Go to a couple of diamond-oriented websites (diamonds.com is a good one), and get a quickie education. Price similar stones here (e.g. a 1-carat brilliant cut, G color, VS-2 clarity), then compare when you get to St. Maarten. I've purchased a considerable amount of jewelry, and usually save 30%-40% over US prices. 14k gold should sell for no more than $10/gram (about $25-$30 here). You also save the sales tax, as there's none in the Caribbean. I do my homework, and it pays off.
Don't fall for the "appraisal" scam. There are all sorts of values placed on jewelry (wholesale, retail, insurance replacement, etc.). If you buy something for $1000 and it's "appraised" at $3000, you haven't purchased it for 1/3 of its value. In fact, the jeweler who sold it to you won't even buy it back for the $1000 you spent. If it sounds too good to be true ....
Congratulations, and have many happy years together. Enjoy your cruise, and buy something nice!!