Credit: BEN STANSALL/AFP/Getty Images; Tim Graham/Getty Images

When Prince William popped the question to longtime love Kate Middleton, he did it with a truly historic piece of jewelry: the same engagement ring that belonged to his late mother, Princess Diana.

Prince William, 28, confirmed the news during a press call in London Tuesday. The platinum ring features an oval blue 18-carat sapphire surrounded by 14 smaller white diamonds.

"It is very special to me," William told press at St. James' Palace Tuesday. "It was my way to make sure my mother did not miss out on today and the excitement that we are going to spend the rest of our lives together."

He added of the engagement: "The timing is right now, we are both very, very happy."

28-year-old Kate, who shall now be known as Catherine, admitted: "[It was] a daunting prospect. Hopefully I'll take it in my stride."

William's father, Prince Charles, gave Diana the same ring when he proposed in February 1981. Back then the piece was originally priced at $65,000 according to the Garrard catalog.

But jewelry expert Michael O'Connor tells the same style ring today would be valued at about $150,000.

"Obviously, Diana's ring is priceless, since it's part of the royal family jewelry collection, but it also has deeper meaning for William and his family because it belonged to his mother," O'Connor notes. "This is a sign of true love since William is showing the world that Kate is worthy of this special ring... It's the ideal piece of heirloom jewelry."

Prince Charles and Princess Diana famously divorced in 1996; Diana was killed the following year in a Paris car crash at age 36.