In fairness, that massive marketing campaign only worked because people are very susceptible to preferring “natural” things over “artificial” things, especially with art and luxury items. Diamonds are not unusual in this regard.
DeBeers' big win was when they convinced the US FTC that synthetic diamonds had to be labeled as synthetic. They're real diamonds, and arguably not different from natural ones. The tests required to detect synthetics keep getting more complicated and expensive as the synthesis technology improves.
The old heat and pressure process was first used by General Electric to make a synthetic diamond in 1956. That's used today to make diamond abrasives by the ton. Making gemstones that way wasn't cost-effective until, in the 1990s, Gemesis, in Florida, got the process to work better. The process is touchy, but that's what computer control is for. DeBeers tried threats and intimidation. Unfortunately for them, the Gemesis CEO was a retired US Army general and didn't intimidate. The synthetic diamond gem industry, based in Florida, was soon going strong.
Then came controlled vapor deposition of carbon. The heat and pressure process forced some metal from the press anvils into the diamond, and this was detectable. The CVD process, using technology similar to that used to make near perfect crystal IC substrates, didn't do that. Synthetic diamonds could not longer be detected with standard diamond industry tests.
DeBeers developed more elaborate tests. Diamonds glow for a few milliseconds after being hit by a UV flash, and DeBeers makes testers which flash the target and then take a picture. The results differ for CVD diamonds, but not by much. There are videos of the difference, and it's now at the point where the precise symmetry of CVD diamonds is what distinguishes them. This blows away a basic sales pitch of the diamond business - the best diamond is a flawless crystal. Now they're up against a semiconductor materials technology that makes perfect crystals. DeBeers has been trying to spin this as "natural flaws" being important, after decades of promoting "flawless" as the goal.
CVD was originally really slow. Then the process got faster. Then more companies started using it. Now there's a glut of small diamonds. You can buy them in kilogram bags on Alibaba.
As for cubic zirconia, that's now so cheap that for $500 you can buy a gemstone 200mm across for about $500.
Most other gemstones have been produced in bulk for decades. You can buy ruby and sapphire rods and sheets, and it's not that expensive. The checkout scanners at Home Depot have a sapphire layer on top of the glass. You can drag tools across those all day for years before they scratch much.
It's funny what massive marketing campaign can do.