Does it cost more to make the artificial diamonds than the artificial diamonds are worth as diamonds?Doesn't matter, the price will come down. (My first computer cost $2k!)
But De Beers is gonna be De Pressed!
De Beers is basically a state sanctioned mafia, they eliminate all competition and rig the diamond market to keep prices high. There's nothing rare about diamonds already, they have many times the amount they allow into worldwide circulation stored in warehouses. They've paid off pretty much every African diamond producing country to force miners to sell all their finds to them so they have control of the whole market. If someone attempts to take diamonds out of many countries without going through De Beers they'll end up in jail or shot. Since they're a monopoly they can control the world supply and keep prices elevated.
Gem quality diamonds can already be made in a lab. You can bet De Beers will never allow that to happen though. They'll pay off government officials to make sure it's illegal, or if necessary they'll do worse to make sure their monopoly is protected.
De Beers is basically the Bill and Hillary Clinton of diamonds. You cross them at your own peril.