Diana received the ring from Prince Charles after the two were engaged in 1981. At the time, the sapphire ring, which is surrounded by 14 diamonds, cost 30,000 pounds, or about $47,500 at today's conversion rate.
"I had been carrying it around in my rucksack for about three weeks before [the proposal]," William told ITV. "Everywhere I went, I was keeping ahold of it, cause I knew if this thing disappeared, I'd be in a lot of trouble."
"It's beautiful," Middleton said, as she showed the ring to ITV's camera.
While Middleton is now wearing Diana's ring, William said, she won't be in his mother's shadow.
"There's no pressure," he said. "Like Kate said, it is about carving your own future. No one is trying to fill my mother's shoes. What she did was fantastic. It's about making your own future and your own destiny, and Kate will do a very good job of that."
William said he didn't know where to start when asked why the couple was apparently so loving, but said she makes him laugh. For her part, Kate said William was very supportive of her.
William and Kate broke up briefly in 2007 but were back together within months.
Prince William is second in line for the throne, after his father Charles, Prince of Wales. Prince William's mother was Diana, Princess of Wales, who died in an auto accident in 1997.
The royal couple plan to live in north Wales, where Prince William will continue his service in the Royal Air Force, the statement said.
ABC News' Nick Watt, Sarah Netter, Jason Volack and Bradley Blackburn contributed to this report.