On Tuesday, the senior members of the British Royal Family hosted a a glitzy Buckingham Palace State Dinner to honour the visit of King Willem-Alexander and Queen Maxima of the Netherlands to Britain.
But when the Duchess of Cambridge stepped out in diamonds and silk taffeta, there was one small but significant detail of her elegant outfit that only the most eagle-eyed royal fan would have spotted.
While all eyes were on the magnificent Cambridge Lover's Knot pearl and diamond tiara she wore to crown her regal up do (and which once famously belonged to Princess Diana), it was the Royal Family Order of Queen Elizabeth II brooch from the monarch that contained the discreet tribute from the Duchess to Prince William.
Duchess Catherine, 36, was given the honour afforded to select female members of the Royal Family, by Her Majesty and wore it in the traditional way pinned to the left shoulder of her pale blue silk taffeta ballgown, designed by one of her favourite designers, Alexander McQueen.
However, according to Majesty magazine, Kate's brooch is different in that it is made out of glass instead of the usual ivory in what is being seen as a touching nod to the passionate conservation efforts made by her husband, including campaigning to end the ivory trade.
As patron of Tusk, the wildlife conservation charity, since 2005, Prince William has previously said: "When I was born, there were one million elephants roaming Africa.
"By the time my daughter Charlotte was born, the numbers had crashed to just 350,000.
"And at the current pace of illegal poaching, when Charlotte turns 25, the African elephant will be gone from the wild."
Knowing her husband's determined stance against the use of ivory, Duchess Catherine reportedly customised the brooch to reflect his work.
WATCH: All the times Duchess Catherine has wowed in a tiara. Post continues...
The brooch itself is a painted miniature portrait of the Queen surrounded by gems and pinned to a yellow ribbon. Other members of the Royal Family to be awarded the honour are the Duchess of Cornwall, the Countess of Wessex and Princess Anne.
It's the second time the Duchess has been seen wearing the brooch since it was first awarded to her last year. The first was at the Diplomatic Corps reception last December.
Kate finished off her regal look with Princess Diana's Collingwood pearl earrings, a diamond bracelet and an ornate pearl necklace, which once belonged to Queen Alexandra.
Other British Royals in attendance at the State Banquet included the Duchess of Cornwall, Prince Charles, The Queen, Prince William and Princess Michael of Kent while British Prime Minister Theresa May was also one of the 150 VIP guests invited.
During the event, held to celebrate Queen Maxima and her husband King Willem-Alexander of the Netherlands' state visit to the UK, the Queen raised her glass and toasted the two royal families' strong ties.