JCK Las Vegas is the jewelry industry’s most important trade show, attracting thousands from around the world every May. Two days before JCK 2018, De Beers—which had held a monopoly on rough-diamond production for most of the 20th century—shocked the industry. The company said that a new subsidiary, Lightbox Jewelry, would begin selling lab-grown diamonds at prices that will undercut competitors, in some cases by more than 75%.

De Beers, formed in 1888 by a group of British businessmen to protect diamond prices, has for decades...