Purple is the color of royalty for a reason.

Kate Middleton attended the London premiere of “Digger” Tuesday, looking radiant in a custom purple long-sleeved Jenny Packham gown.

The design featured sheer, voluminous sleeves and the Princess of Wales, 44, pulled her hair into an elegant half-up style that cascaded into waves and showed off her beloved Collingwood pearl drop earrings.

But the real star of her ensemble might’ve been the Queen Mother’s Amethyst Sautoir Necklace, a dazzling piece of jewelry that has rarely been seen in the public.

Kate Middleton wore the Queen Mother’s Amethyst Sautoir Necklace to the “Digger” premiere. / SplashNews.comShe looked radiant in a bright purple Jenny Packham gown. ZUMAPRESS.comShe mingled with the film’s stars — including Tom Cruise — on the red carpet. Andrew Parsons / Kensington Palace

The necklace features of pearls alongside diamonds and amethyst, all framing a heart-shaped gemstone pendant. It was a wedding gift from Queen Alexandra to Elizabeth Bowes-Lyon, given to the future Queen Mother in 1923.

The precious piece is one royal jewel that’s oft left locked up. It was last worn publicly by Queen Camilla at the 2012 premiere of “Skyfall.” Like the Princess of Wales, she also chose to match her outfit to her necklace, wearing a violet velvet long-sleeved dress.

Husband Prince William chose a dapper velvet tuxedo jacket for the red carpet appearance. / SplashNews.comMiddleton wore her hair in an elegant half-up ‘do, showing off her pearl drop earrings that once belonged to Princess Diana. / SplashNews.comThe necklace has rarely been seen in public; Queen Camilla is the last royal to bring it out of the vault, sporting the bauble with a velvet purple gown at the 2012 “Skyfall” premiere. Getty Images

Middleton was in attendance with husband Prince William, who wore a black velvet jacket and bowtie. The couple mingled on the red carpet with the cast, including Tom Cruise, Jesse Plemons and Emma D’Arcy.

Also in attendance were Salma Hayek, Jodie Turner-Smith and Hannah Waddingham.

This is the first red carpet appearance for Middleton in months; she previously attended February’s BAFTA Awards in a blush Gucci gown.