Diamonds International, as previously mentioned, is not owned by Carnival Corp.
If it was, other cruise lines would not be promoting DI to their passengers.
Starboard is also not owned by Carnival Corp. If it was, there would not be Starboard shops on non-Carnival Corp ships.
Starboard and DI are direct competitors for passenger dollars, and as such, are bitter enemies.
PPI, which does have financial interests in DI, is a bit of a scam artist. Their "front" is Porthole Magazine, which (if you pay attention) has little but advertisements for all the shops (like DI) that are promoted by the onboard shopping ambassadors (who work for PPI).
PPI also gives all sorts of impressive "awards" to the cruise lines that carry those shopping ambassadors on their ships.
The onboard shopping ambassadors work for PPI.
PPI gets paid by the cruise lines to have the shopping ambassadors onboard.
PPI gets paid by the shops (like DI) to have the shopping ambassadors onboard, promoting the shops ashore.
All of this has been very successful. PPI has been able to convince feeble-minded cruise passengers that gems which do not come from the Caribbean or Alaska, sold in shops run by East Indians, are somehow far cheaper to buy there.
Has anyone here seen any Diamond Mines in the Caribbean?
In reality they are not cheaper. Typically you can get a better deal at home or on the internet - and from a reputable dealer who will really stand behind the purchase.