How do the super rich like to buy their diamonds? It’s a question that weighs heavily on the minds of top-tier jewellers. Wandering along Bond Street or Fifth Avenue, gazing agog at the sparkliness on show is all well and good for Holly Golightly and co, but for many UHNIs discretion is key. It simply won’t do to be seen mulling over a fortune’s worth of jewels in plain view – and smartphone distance – of celeb-spotting tourists. Hence the proliferation of “private buying suites” lining Bond Street’s back offices – but there’s nothing sexy about a desk in a dark room, no matter how precious the goods on show.

All of which is why a new breed of jewellers are opening private, by-appointment-only ateliers that offer the utmost in discretion – not to mention negating the need for off-puttingly burly bouncers. One such is Star Diamond where customers are whisked into an airy, contemporary art-filled apartment above Mayfair’s Dover Street and invited to plonk themselves on a plush sofa while the chic staff offer up some of the world’s rarest gems for their perusal.

Star Diamond began life as a Hatton Garden diamond trader in 1948. Specialising in coloured diamonds, it opened its long-awaited London gallery in April 2014, in response to clients’ demands for it to set, as well as sell, the stones. And set them it does – not just in predictable solitaire rings but in an array of daring, contemporary designs. Zingy cocktail rings pose next to exuberantly large diamond hoops, while a pair of Firework earrings explode with a rainbow of mismatched but equally vibrant coloured fancy vivid stones; a strikingly modern way to spend a cool £1 million.

Despite the fact that its diamonds sit in the top two percent in terms of rarity and quality, at Star Diamond there’s a refreshing lack of reverence towards the gems. Staff don’t pore over them with a loupe, discussing carat weight and clarity – although well they might, given their mind-boggling worth. As I try on the sculptural ring worn (and Instagrammed to rapturous response) by Ellie Goulding, its diamond tendrils snaking around a pear-shaped white and a vivid yellow diamond, Marlene Goldsmith, Star Diamond’s head of UK retail, is already talking about having it re-set. “It’s unusual, a real statement piece,” she says. “But if you get bored of it you can always take it apart and have the stones set in something else.” How many other jewellers speak so freely of dismantling their precious creations?

Shopping might have an informal air, but the pieces housed in the atelier are for serious diamond connoisseurs. Australia’s Argyle mine, which produces over 90 per cent of the world’s pink diamonds, is set to close in the next five years, drying up supply and making the rare stones even more desirable – and a sure-fire investment. Star Diamonds co-founder Andre Abadjian would jump at the chance to buy back a pink diamond piece at well above its original retail price, I’m told, safe in the knowledge that its value is set to soar.

The house’s Masterpieces collection showcases these rare stones in one-of-a-kind designs such as the Giverny bracelet – a collection of 259 pastel-hued diamonds, totalling 125.77 carats, collected over the course of two years and positioned to mirror the colour of Monet’s gardens at Giverny; or the Diamond Blossom ring which celebrates an extraordinary peach-coloured 32.21-carat oval diamond. And for those with particular taste, the house offers a bespoke service, using modern CAD technology to produce 3D-printed wax moulds of unique designs, which are tweaked and perfected before being cast in metal and mounted with their show stopping stones.

A contemporary approach and a refreshingly unstuffy attitude towards the world of fine jewels: for modern day Elizabeth Taylors, this hidden Dover Street boutique could be the North Star.