The late Princess' three-stone engagement ring is worth almost 10 times more today than in 1956Grace KellyCredit: Bettmann / Getty; Art Selby/NBCU Photo Bank/GettyPrincess Grace had two magnificent engagement rings, but the second is, perhaps surprisingly, more famous than the first.Just months after Grace met Prince Rainier III of Monaco during a photocall at his palace, he proposed to her over the 1955 Christmas holidays at her family home in …