I've never bought anything other than costume jewelry on a cruise ship. I haven't seen any real jewelry (14k or better) on the ship that couldn't be found at home for better price/quality.

Also haven't bought much jewelry in ports, with two exceptions:

I do scour the Mexican ports for sterling silver pieces. Previously I could find many items for $1/gram but I guess those days are long gone now, with the price of silver going up as it has. I always ask the gram weight.

Also, I purchased a set of Movado watches (his and hers, museum design) in a high-end jewelry store in Nassau and it was essentially a buy one, get one free deal. I've been very happy with those. It was definitely an offer I am glad I took advantage of. And I have seen similar Movado offers in other ports since then.