Kate Middleton wears the famous engagement ring of Diana, Princess of Wales

Prince William proposed with the blue sapphire and diamond ring that his father,  Prince Charles, presented to Princess Diana for their engagement.
 Prince William explained his thinking during the press conference to announce the couple's engagement:

"It's very special to me. As Kate's very special to me now, it was right to put the two together."

Kate Middleton wears the engagement ring of Diana, Princess of Wales, as she poses for the media with Britain's Prince William following the announcement of their engagement, at St. James's Palace in London, Tuesday Nov. 16, 2010. The couple are to wed in 2011