When Zendaya took to the Golden Globes carpet this year with a very large and very sparkly engagement ring, the internet was soon abuzz with questions over who had designed it.

It wasn’t Bulgari, the Italian jewelry house for whom Zendaya is a brand ambassador. Nor was it a big name brand owned by a luxury conglomerate. Instead, the ring, a 5.02-carat diamond in a slightly quirky Georgian-style setting, was the work of Jessica McCormack, a New Zealand-born designer who has become a go-to for cool “day diamonds” by the likes of Dakota Johnson and Dua Lipa — and a growing fan base of wealthy women who don’t believe their sparkles should languish in the safe.

“What’s the point in that?” McCormack said last week from an armchair in her first store in the United States, which opened Tuesday on Madison Avenue in Manhattan. “There’s no joy if you can’t see them and others can’t enjoy them on you.”

McCormack, who speaks with a friendly Kiwi twang, wore a fuzzy gray cardigan, old deck shoes, white socks with red hearts and diamonds.

Lots of them, in fact, shimmering on her ears, neck and wrists as workers put the finishing touches on the wood-paneled beaux-arts town house designed by restoration architectural firm Johnston Cave Associates.

“My whole idea is of relaxed ease and fitting into your daily life,” she said. “It’s about functionality and fantasy, making jewels that are fun and cool but wearable and that just make you incredibly happy every time you look in the mirror.”

McCormack had no formal design training. But her father, an art and antiques dealer, helped hone a magpie’s eye for vintage treasures and craftsmanship. There followed a stint in Sotheby’s fine jewelry department in London, where her appreciation for vintage pieces grew as she handled Russian crown jewels alongside Cartier and Lalique pieces from the 1920s.

After some experimentation designing pieces in an antique-meets-modern aesthetic, she started her namesake brand in 2008, with debut pieces like her Wings of Desire earrings, a sweep of graduated diamonds that climb up the ear and caught the eye of Rihanna, who became one of her first customers.

Other cult pieces, spotted on Zoë Kravitz, Rosie Huntington-Whiteley, Adele, the Duchess of Sussex, Victoria Beckham and Margot Robbie, include her Gypset earrings, little hoops with suspended Georgian-style button-back diamonds, sapphires, emeralds or rubies that are highly covetable (and widely imitated), with prices starting at $4,550 a pair.

Her Ball N Chain necklace can be layered with assorted diamond pendants in an array of cuts and prices. Some could come from McCormack’s latest Fruit Salad collection — that is, peaches, pears, apples, cherries and lemons made out of intricately set pave gemstones and 18-karat gold.

The pieces strike a balance between looking at home on the red carpet and being trendy, easy-breezy accessories to wear for drinks or on the school run (albeit for the rarefied few who wouldn’t think twice about wearing a 7-carat rock with a T-shirt and jeans).

“The appeal of Jessica McCormack’s pieces is their distinct unfussiness coupled with plenty of charm and fun,” said Daisy Shaw-Ellis, American Vogue’s jewelry director. “She has modernized antique and vintage designs and pushed the boundaries with expected materials without losing their integrity.”

A major weapon in McCormack’s arsenal are her investors.

Rachel Slack is of the Oppenheimer dynasty, and Michael Rosenfeld is a third-generation diamantaire. Last year, Lingotto, an investment firm owned by the Agnelli family’s holding company, Exor, also took a stake.

They all hail from family businesses that understand the eccentricities and complexities of the long game in luxury jewelry, unlike many venture capitalists who often invest in brands to make a quick profit, only to be left sorely disappointed.