While Pippa's engagement ring is slightly smaller than her sister Kate's - and carries a lot less history - it cost much more.

Believed to be made to order, it is very much in the Art Deco style of the 1920s and 1930s.

Experts say it is almost certainly made in platinum and has a three-carat Asscher-cut - or square-cut - diamond.

Kate's ring would cost less than Pippa's to make, but its history makes it priceless. Photos / Getty, Snapper media
Kate's ring would cost less than Pippa's to make, but its history makes it priceless. Photos / Getty, Snapper media

This is surrounded by smaller round brilliant diamonds in a rub-over or bezel setting, in which the stones are held in place with a rim of metal, a more secure arrangement than the traditional setting and ideal for a sporty young woman like Pippa.

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Kate's ring was inherited by William following the death of his mother in 1997.

It comprises a 12-carat Ceylon sapphire surrounded by 14 solitaire diamonds set in 18-carat white gold.

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When Prince Charles bought it for his engagement to the then Lady Diana Spencer in 1981, it cost £28,000. It would be worth £108,000 today. However, the history attached to the piece and the scarceness of the sapphires - not to mention the fact that it is unlikely to ever come on the open market - makes it almost priceless.

David Allen, creative director of 77 diamonds, said: 'If you were going to buy a ring tomorrow and wanted a ring like Kate's or Pippa's, Pippa's would be pricier.

"It's only the history and story behind Kate's ring that makes it more valuable."

The Asscher cut is named after its creator Joseph Asscher. The Dutchman was one of the greatest diamond cutters of the early 20th century. His style has undergone a revival in recent years.

The most famous Asscher-cut diamond was worn by Hollywood screen legend Elizabeth Taylor - the 33.19-carat Krupp diamond bought for her by her then husband Richard Burton in 1968. He paid £150,000.

Elizabeth Taylor wearing the 33 carat diamond ring in 2006. Photo / Getty
Elizabeth Taylor wearing the 33 carat diamond ring in 2006. Photo / Getty