Kate sparkled in diamonds as she and Prince William stepped out for the annual winter party at the palace

Kate Middleton sparkled in diamonds as she and Prince William stepped out for the annual winter party at the palace Tuesday.

The royal couple, who are expecting their third child in April, left children Prince George and Princess Charlotte at home to attend Queen Elizabeth’s reception for diplomats at Buckingham Palace. It is one of the largest parties the queen hosts each year.

Kate was pictured being driven into the palace wearing the Queen Mary’s Lover’s Knot tiara (often also called the Cambridge Lover’s Knot tiara), which was a favorite of Princess Diana. Her classic updo showed off diamond and pearl drop earrings and a diamond necklace. Kate also wore the stunning headpiece to last year’s reception.

Diplomatic Reception at Buckingham Palace
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Princess Kate attends Queen's Diplomatic Reception
Credit: Andrew Parsons/i-Images/Polaris

A little more than century old, the tiara was originally commissioned from famous court jewelers Garrard by Queen Mary sometime in 1913 or 1914. It’s actually a replica of a 200-year-old Gothic Revival tiara owned by Mary’s grandmother, Princess Augusta of Hesse, the Duchess of Cambridge. (Hence the alternate name of “Cambridge Lover’s Knot.”) The original Cambridge Lover’s Knot Tiara was later sold at auction at Christie’s in Geneva in May 1981 for more than $747,000 in today’s dollars to an anonymous buyer and is presumed to be in an unknown private collection.

Princess Kate attends Queen's Diplomatic Reception
Credit: Andrew Parsons/i-Images/Polaris
Buckingham Palace Diplomatic Dinner, London, UK - 05 Dec 2017
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It is the fourth time she and William have attended the party, which sees diplomats from more than 170 missions in the U.K. greeted by members of the royal family. Kate first attended the annual reception in 2013, where she wore the famous Lotus Flower tiara.

The Duchess Of Cambridge is seen wearing a Tiara as she attends a diplomatic event at buckingham palace.
Kate Middleton in 2013
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Also on hand to help the Queen entertain guests was Prince Charles and Camilla, Duchess of Cornwall, who wore the late Queen Mother’s Greville tiara, which was named after a socialite who gifted it to her in 1921. The height of the tiara was increased in 1953 by adding more diamonds. After the Queen Mother passed away in 2002, the tiara went back to the Queen who frequently loans it out to Camilla.

The Queen's Diplomatic Reception
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The white-tie bash heralds the beginning of the festive season at the palace. Later in the month, the Queen will host her extended family for a pre-Christmas lunch before heading off to her estate in Norfolk, Sandringham.

Queen Elizabeth II
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Last year, the palace released a picture of the senior members of the family lined up wearing their jewels and medals (and sashes and fancy breaches!) before the party started.