As a general rule of thumb I would say never buy expensive jewelry outside the states as you have no recourse to return said items except for an expensive flight.
However, if you sailed on Carnival and did as instructed and showed your purchase to the shopping guide you could have guaranteed them. DI is a Carnival approved merchant and as such your purchase can be guaranteed by Carnival to be worth what you paid for it. Most people don't realize though that for that guarantee to mean anything you have to have their shopping guide/specialist see the purchase and "log it it".
I think that DI is also an approved merchant with Princess and Royal but I have no idea how their guarantee works.
If you have an"appraisal" from DI saying they are .75 carats and you get a certified gemologist (not just a jewelry shop) to appraise them at .25 carats I would be contacting Carnival. a half carat is too big of a difference for them to not know they were .75. Maybe Carnival can do something (like help you get a refund)
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007