This is interesting to note. Every cruseline that I know of recommends Diamonds International, in fact they tend to promote them incessently.
Obviously, there's a kick back arrangement between DI and the CL. At the same time though, I would expect some sort of security or protection in making such a large purchase. Most cruiselines provide a 30 day guarantee on things you purchase at their recommended stores. Check to see if this applies to you.
We bought jewelry at some smaller recommended stores and found we got items for about 1/2 price. From reading this posting, it sounds like the smaller unrecommended stores might be ok too - but as always it's buyer beware.
Good luck with your refund!
RCI Sovereign of the Seas 2005
Caribbean Princess 2005
Carnival Paradise 2004