Early in life, such design was not on Mr. Gruosi’s mind. He was born in Beirut, Lebanon, and his Italian mother brought him to Florence after his father, who was Lebanese, died. As a teenager, feeling fully Italian, he took his mother’s maiden name and finished high school. But after getting married at 18 and with a baby on the way, he took an entry-level job at a local jewelry boutique. “I was in the atelier every day — looking, checking, asking questions,” he said, adding that the owner had taken notice and sent him, at 25, to open an outlet in London.
By the time he was 40, after working at Harry Winston in Saudi Arabia and at Bulgari in a traveling global role, he and two partners had begun De Grisogono with 16,000 Swiss francs, the equivalent of about $33,000 at the time. In 1996 he bought out the partners with the financial backing of his third wife, Caroline Scheufele of the Chopard family, and then in 2014, he bought out Chopard’s 49 percent stake.
Black diamonds put De Grisogono jewelry on the global map in 1996, the novel gemstone capturing female buyers’ imaginations. “Sales took off,” he said, allowing the brand to open stores in London and Gstaad, Switzerland. (Today, there are 140 points of sale, including authorized retailers.)
In 2000, square-cut black diamonds were featured in De Grisogono’s first watch, the Instrumento Uno for men, which had a blackened steel movement and a tonneau, or barrel, case inspired by the shape of a TV set. “I told my staff if we sell 30 at Baselworld, we’ll make a party,” Mr. Gruosi recalled, referring to the annual watch fair in Switzerland. “Well, we sold 876!” The women’s version, the Instrumentino, debuted the following year.
The 2008 financial crisis was a turning point for the company. For years De Grisogono’s production had been divided evenly between jewelry and watches, but afterward, watch production was slashed. “I had to try to save my company, and the only way was to cut costs,” Mr. Gruosi said. “Watches take much longer to develop and produce — suppliers must be paid in advance, dials take 12 months, movements take one to two years.” Watches now represent 15 to 20 percent of the company’s output, but he plans to return to an even split in the next two to three years.
Mr. Gruosi, 64, continues as the house’s creative director, leaving his mark on every design. In watches, there are 25 collections — 7 primarily for men and 18 exclusively for women.
Although he never trained as an artisan, he expects perfection from his employees and does not hesitate to tell one to start over — to reset, for example, the stones on the dial on a ladies’ Grappoli watch — if he doesn’t like the way it looks or feels.
He paused at one setter’s desk to pick up a half-finished Grappoli, its pavéd dial surrounded by a circle of sparkling pink-sapphire briolettes. The shimmering pink band, made from stingray skin, called galuchat in French, would be attached later. “My idea for the Grappoli came to me while I was in a vineyard in Tuscany two years ago,” he recalled, referring to his favorite style of the highly ornate wristwatch, set with emeralds. “I’m in love with green. My influence always comes from what I see around me — the sky, the sea.”
Mr. Gruosi acknowledged that he works constantly and usually feels stressed. He spends much of his time on the road, often entertaining clients who quickly become friends, but also is sure to clock seven weekdays a month at the company headquarters. He said he is moving his main residence from Geneva to London, reflecting the importance of the house’s store on fashionable New Bond Street.
The highlight of the year? The lavish party that Mr. Gruosi throws during the Cannes Film Festival so celebrities can showcase De Grisogono jewels for about 650 well-heeled clients, or potential clients, amid a flurry of flashing cameras. “People are not scared to come,” he said. “Sales people don’t go up to them. We are different.”
Mr. Gruosi checked his own watch, a rectangular New Retro, the throwback ’50s-style timepiece that the company introduced at Baselworld last year. It sells from $30,000 to as much as $500,000 set with baguette-cut stones. “I was watching TV in my apartment here and the design came to me almost two years ago,” he said, adding that it also was inspired by a cigarette lighter.
The company also is busy marketing its first smartwatch collaboration, the Samsung Gear S2 by De Grisogono, a jeweled timepiece meant to symbolize the best of two worlds that sells for $16,100. “No one knows it’s a digital watch until you swipe it,” Mr. Gruosi said, seeming keen to embrace technological innovation to gain a competitive edge. “I’m very happy with it. There will always be people buying traditional watches. But the world, a new generation, is going towards digital watches.”
As for his next watch, the mastermind remains coy. “I’m still looking at what I can see around me,” he said. Once he sets his mind on an idea, he said, the process moves quickly. “I don’t have a bunch of people — there are three or four of us — so we can decide in an hour, or the next day.”