Kate, 40, was seen wearing the late monarch's diamond leaf brooch with a trio of pearls, which Queen Elizabeth II wore during a concert on her 73rd birthday in Seoul, 21 April 1999.
Kate has worn the brooch before: she first wore it in July 2017 at the Tyne Cot Commonwealth War Graves Cemetery in Belgium, as part of a series of commemorations for the 100th anniversary of the Battle of Passchendaele.
It was no doubt carefully selected as 'white' jewels are thought to be worn for mourning, with her wearing it again for the annual Festival of Remembrance at Royal Albert Hall in 2018, marking the centenary of the end of World War I.
While it is unknown who exactly originally owned the brooch, it is thought to be a fairly recent creation, as described by The Court Jeweller, with the late monarch loaning it to her daughter-in-law.
Diamond experts at UK retailer Steven Stone also detail that the pearls are placed in the centre of a pave-set leaf, with close up shots suggesting the piece appears to be made partly of yellow gold. They have valued the piece at approximately £60,000.
During the procession on Wednesday, Kate also wore Diana, Princess of Wales' Collingwood pearl drop earrings, as she did at the event in Belgium.
Meanwhile, Meghan paid tribute to the late Queen by wearing the pearl and diamond earrings she gifted her in 2018, before their first engagement together at Mersey Gateway Bridge in Cheshire.
The monarch also loaned her the Queen Mary's diamond bandeau tiara from her personal collection to wear at her wedding to the Duke of Sussex on 19 May, 2018.
Kate has adopted many jewels over the years. Most notably, these include a diamond necklace first gifted to the Queen by the Nizam of Hyderabad which Kate wore at London's National Portrait Gallery, and a Cartier Halo tiara she wore for her wedding to William, first gifted by King George VI to the Queen Mother who then passed it down to her daughter.
The procession transported the Queen's coffin from Buckingham Palace, starting at 2.22pm, to Westminster Hall in the Houses of Parliament to lie in state.
Prince Harry and William walked behind the coffin along with their father, uncles and aunt while Kate and Meghan travelled by car. Kate sat in the same car as the Queen Consort while Meghan accompanied Sophie, Countess of Wessex.
Kate first met the late Queen at the wedding of William's cousin Peter Phillips to Autumn Kelly on 17 May, 2008, although William wasn't there himself.
Meghan opened up about when she officially met her husband's grandmother during her interview with Oprah Winfrey, which she said occurred in Windsor after the late monarch finished a church service. The duchess memorably recalled it involved some last-minute curtsey practice.
Watch: Rehearsals underway for Queen's funeral