William told reporters he gave Kate the ring to be sure 'my mother didn't miss out on today and the excitement.' The distinctive ring, which cost £28,000 in 1981, features a blue sapphire surrounded by 14 diamonds.
Diana stopped wearing the stone after splitting from Prince Charles in 1996.
William and Kate will give their first joint interview tonight on Sky News.
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Is it unlucky for Kate Middleton to wear Princess Diana's ring?
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