If this is true, then the Soviet Union must have come up with an alternate method to produce "tons" of these high quality, large diamonds long before anyone else:
https://en.wikipedia.org/wiki/Synthetic_diamond
The two methods then known to make synthetic diamonds, were only useful for industrial diamond purposes, if I'm reading that correctly. It seems like another alternative process wasn't created until the 1990s. It also seems like none of these (?) known processes are useful for gem quality diamonds, or if they are, they can't make such diamonds of large size in large quantities.
If the then-Soviets had this technology - and still have it - it would be a massive disrupter to the entire diamond trade business. I would imagine that even for industrial diamonds, it would still cause problems (you know, problems like lower prices and better availability).