The Princess of Wales said her family miss Diana every day in an emotional heart-to-heart with a well-wisher.
She stopped to speak to the fan yesterday, during her two-day trip to Wales with Prince William, when Diana came up in conversation.
The woman complimented Kate's famous engagement ring, which originally belonged to her mother-in-law, resulting in the Princess sharing a new bit of information about the stunning piece.
Kate replied: "It's the same ring and it was exactly the same size when I tried it on."
She continued: "With the children and the grandchildren, she would've loved being a grandmother so much. She'd be brilliant. We miss her every day."
Laura-Ann Barr posted a clip of the exchange to social media, and captioned: "Kate talking about Diana yesterday... and her engagement ring was exactly the same size."
The exchange came as William and Kate popped into a local rugby club after spending the afternoon with mountain rescue workers. During the outing, they abseiled down the side of a cliff in the Brecon Beacons.
Afterwards, they made a trip to the Little Dragon pizza Van, where the princess spoke with members of the public.
The video was posted this morning, as William and Kate make their visit to the Welsh village of Aberfan - the site of the 1966 disaster in which 144 people, including 116 children, were killed.
The couple were led through the Aberfan Memorial Garden on Friday to pay their respects to those who died when a colliery spoil tip collapsed and sent tonnes of ash slurry onto the village below.
The garden sits on the site of former Pantglas Primary School, which was engulfed by the landslide at 9.13am on October 21 1966, just as lessons had begun.
People gathered on the street to greet the royal couple as they arrived.
They were guided around the memorial garden by Aberfan survivor David Davies, a former pupil at Pantglas Primary School, and Professor Peter Vaughan, Lord Lieutenant of Mid Glamorgan.
They met trustees from the Aberfan Memorial Trust who are involved in ensuring the maintenance of the garden, alongside some of the Aberfan Wives group who lost relatives in the disaster.
It is the first time the royal couple has been to the village near Merthyr Tydfil in South Wales.
Queen Elizabeth II visited eight days after the tragedy and shortly after a mass funeral had taken place.
Her decision not to visit sooner was said to have been one of the greatest regrets of her reign.
Despite this, mourners gathered to meet her and the bond she made with those she spoke to during that visit on October 29 1966 endured throughout the rest of her life.
In 1974, she opened the garden to commemorate the victims and provide a place for bereaved families, survivors and the community to pay their respects.
She visited again in 1997, with her final trip to Aberfan in 2012 when she opened a new school called Ynysowen Community Primary.
On the 50th anniversary of the disaster, her son Charles - then the Prince of Wales - visited the village and delivered a message from the Queen.
In it she said: "I well remember my own visit with Prince Philip after the disaster and the posy I was given by a young girl which bore the heart-breaking inscription 'from the remaining children of Aberfan'.
"Since then we have returned on several occasions and have always been deeply impressed by the remarkable fortitude, dignity and indomitable spirit that characterises the people of this village and the surrounding valleys."
