My rule of thumb is never pay more than what I would pay if it were costume jewelry. I have no idea whether a stone is real or fake or what the value of it is. If I like it and I would pay that price for fake, I buy it, otherwise, I pass. (I am not suggesting that the jewelry is in fact fake.)

As someone else has said everything on a cruise is a profit center. If the jewelry is as overpriced as everything else, I'd suggest saving your money.