“Invention,” Gene Wilder’s Willy Wonka famously said in the classic 1971 film, is “93 per cent perspiration, six per cent electricity, four per cent evaporation, and two per cent butterscotch ripple”. Now that the chocolate factory is under the purview of a new Wonka, the always fabulously dressed Timothée Chalamet, that recipe looks rather different – if no less fantastical. Formulating his latest internet-shaking red-carpet look, for example, called for one pink velvet Tom Ford tux, one historic Parisian jewellery maison, a sprinkling of vintage references and 450 hours of painstaking work.

The result? The one-of-a-kind, candy-inspired Cartier necklace the modern day matinée idol wore to walk the red carpet at the world premiere of Wonka at London’s Royal Festival Hall tonight. “It’s insane, right?” Chalamet says admiringly of his new accessory, which was fitted precisely to the contours of his throat in order to sit directly against the skin, allowing all 964 of its emeralds, rubellites, pink tourmalines and blue opals to shine. “It’s beyond anything I could have dreamt,” the 27-year-old tells Vogue a day before the screening.

Chalamet, a house ambassador who has been known to wear metallic Tom Ford tailoring with a classic Juste un Clou bracelet, or board shorts with a vintage Tank watch, spent hours combing through archive Cartier imagery before travelling to Paris to meet with Marie-Laure Cérède, creative director of jewellery and watchmaking, at the atelier months before the premiere. “It seems like there was a really expressive period in the ’60s and ’70s when Cartier creations were quite colourful, and really playful, even,” he says of what he took away from his research. “The house has such success with their staples… my one curiosity was what something joyful and playful and youthful would look like under the umbrella of a Cartier creation.” For Cérède, the challenge was to create a unique piece that “fits Timothée”, but was simultaneously “so Cartier that it couldn’t have been done by anyone else”, she explains.

The necklace’s intense pink, vibrant green and milky blue stones certainly reflect Chalamet’s joyful brief, and set the piece apart from the more muted jewellery (a discreetly expensive watch here, a vintage brooch there) we traditionally see men wearing on the red carpet. But then, Timothée has never had any time for “rules” around what men and women should or shouldn’t be wearing. “Absolutely,” he replies emphatically, when I ask if he takes the same genderless approach to jewellery as he does to fashion generally. “Maybe it’s having gone to a performing arts high school in New York, or maybe it’s just the way my mom raised me,” he says. “I’ve always just worn what fits.”

For Chalamet, as we know, “what fits” could extend to a backless Haider Ackermann jumpsuit, a Louis Vuitton harness or, most recently, a lilac Latex Prada suit. Still, the actor insists his tastes are often surprisingly simple. “I’ve always just loved a thin chain,” he says. “You know, even one of the necklaces I wear in Call Me By Your Name… the Star of David is obviously from the book, but another was one I just happened to have bought in Milan one weekend leading up to the movie. Luca [Guadagnino, the film’s director] kept it in.”

That spontaneous purchase, since immortalised on screen in the film that made him one of Hollywood’s most bankable young stars, is now “tucked away at home”, says Chalamet, who adds that all of his most treasured pieces are those with memories attached. “I’ve always just loved jewellery with a meaning, whether that’s a wired bracelet from a special trip, or something intimate I would have received from a family member.” To the actor, “jewellery or fashion or anything you hold dear matters much more because of the sentiment behind it, or the feeling it gives you, than the price tag, or how sparkly it is.”

Timothée’s new Wonka necklace manages to be both sentimental and sparkly, given that it references perhaps his most major role to date, and combines a white gold base with close to 1,000 precious stones in a rainbow of sweetie shades. The distinctive colour combination – very pale blues mixed with strong pinks – is deliberately unexpected, Cérède explains. “It’s not very obviously harmonious,” she says. “It’s very [contrasting colours], which is quite audacious I think.”

Unlike the chain Chalamet wore as Elio in Call Me By Your Name, he won’t be tucking this one away in a drawer at home once his Wonka promo wraps: after tonight the spectacular piece will become part of the Cartier Collection, which goes on display at jewellery exhibitions around the globe. Of course, he could always put it on his Christmas list. While the maison declined to put an exact price tag on the necklace, his rumoured girlfriend Kylie Jenner is a self-made billionaire at 26 – and a fan of a Cartier Love bracelet herself.

But the actor insists he’s not holding out for any gifts in particular this year. “You know, Christmas is very close to my birthday on December 27th. I try to keep my expectations low.” Now that the laidback star is linked to a Kardashian-Jenner – a family that never misses an opportunity to commission an elaborate flower wall or balloon arch – he might just find himself surprised.