TI is a part of Diamonds International. Their customer service is located in NYC.
That is their policy. You have 30 days to return the jewelry if an independent appraiser states the stone is of lower quality than they sold it. There is no room for buyer's remorse. Check your receipt. The Carnival buyer's protection program is really no better.
You might be able to work with your credit card company. If you send it back use some trackable method.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015