The Princess of Wales, 41, stopped to talk to members of the public during a walkabout in Wales.
Diana tragically died in a car accident in 1997.
A clip posted to Instagram shows Kate speaking to fans about her blue sapphire ring, which originally belonged to her late mother-in-law.
“It’s the same ring and it was exactly the same size when I tried it on,” she is heard telling a fan.
She adds: “With the children and the grandchildren, she would’ve loved being a grandmother so much.
“She would be brilliant. We miss her every day.”
It came a day before the Prince and Princess of Wales visited the town of Aberfan, where a landslide killed 144 people including 116 children in 1966.
William and Kate were given a tour of the Aberfan Memorial Garden, opened by the Queen in 1974 to commemorate victims.
The royal couple were greeted by cheering crowds waving Welsh flags as they arrived at the village.
They were introduced to Mair Morgan, one of only four teachers to have survived the disaster and the only one still alive today.
Earlier this week, the Royal couple abseiled together off a cliff in the Brecon Beacons, while meeting mountain rescuers.
William and Kate arrived at a wet and windy area of the Bannau Brycheiniog near Merthyr Tydfil in the South Wales Valleys on Thursday wearing hiking gear, and put on safety equipment ready for the descent.
They were guided through the process by the Central Beacons Mountain Rescue Team, a volunteer-run organisation which responds to calls 24 hours a day, all year round.
