The necklace was paired with a bracelet and matching pair of earrings, and are part of the famous 'York Diamond Demi-Parure', a gift from Queen Elizabeth and Prince Philip on the occasion of her wedding to Prince Andrew.
The 'York Diamond Demi-Parure' is a set bought by Her Majesty at famed London jeweller Garrard, along with the York Tiara.
Fergie and Prince Andrew - who share daughters Princess Beatrice and Princess Eugenie - were married in 1986 at Westminster Abbey, however, the couple went through a very public and messy divorce less than 10 years later, finally separating in 1996.
Since then the pair have been known to be the friendliest exes in the royal family and are frequently spotted together.
While the duo have always been friends, the public displays of affection have ramped up in recent times with many believing a rekindled romance is imminent.
Considering the royals never do things 'by accident', Fergie wearing her wedding set of jewels out in London may be taken as a very public tribute to Prince Andrew, as for what that could mean, only time will tell!
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