In addition, Mr. Coe was also at pains to debunk what he called many of the “misleading and bogus claims” around synthetic diamonds: that they are more sustainable alternatives to mined stones, with shorter supply chains and smaller carbon footprints.

“Given the pressure required to create lab-grown diamonds, it’s akin to the Eiffel Tower being stacked on a can of Coke,” he said. “If you look at the detailed numbers, the energy consumption levels between natural and man-made diamonds are in the same ballpark.”

This is not the first time De Beers has created brands and advertising strategies in response to disruption in the diamond market since it gave up its monopoly in 2000, abandoning its 60-year policy of controlling supply and demand to concentrate on mining and marketing instead.

In 2002, after fashion brands such as Dior and Chanel began seriously penetrating the fine jewelry market, selling the importance of their design expertise, De Beers entered into a joint venture with LVMH Moët Hennessy Louis Vuitton and founded De Beers Diamond Jewelry. (De Beers had been forbidden to directly sell or distribute its diamonds in the United States because of longtime antitrust issues, since settled.) In 2017, De Beers bought out the 50 percent stake owned by LVMH to take full control of the brand.

Owning the brand gives De Beers “a much better view on what you think people will pay for medium- and long-term supply,” Mr. Cleaver said. “It’s an exceptionally valuable business for us in that sense. So is Forevermark.” That brand, which focuses on responsibly sourced gems, was created in 2008, partly in response to consumer appetite for conflict-free diamonds.

Lightbox is fully in line with this strategy. “Synthetics are fun and fashionable, but they are not real diamonds in my book,” Mr. Cleaver said. “They aren’t rare or given at life’s great moments. Nor should they be.”