The stunning 178-diamond and pearl necklace Princess Diana wore at her last official engagement -- the Royal Gala Performance of "Swan Lake" at the Royal Albert Hall on June 3, 1997, three months before her death -- was unveiled in New York yesterday prior to its sale. The piece, along with a pair of matching earrings that were made for Diana but never worn, is set to be auctioned off Dec. 16 by Guernsey's. The minimum opening bid is $500,000, and while there is no estimate on what the set might finally fetch, the sky seems the limit.