The general opinion is this is a synthetic diamond laundering operation. Synthetic diamonds are of course chemically identical to natural, but natural diamonds still fetch a higher price.
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).
The other technology you mention is CVD (chemical vapor deposition), which was not developed in Russia, and was not commercially viable for gem-quality diamonds until the last decade either.
I believe the point being made is that the only ones publically admitted to being lab made were the industrial grade. The lab made gem grade were made to look like mined stones. The belief is the Soviets kept this a closely guarded secret because it was very profitable for them.
If a synthetic version is better then the natural version can you still call it synthetic?
You are right though, gem-quality synthetic diamond technology didn't come around for several decades after this mine started producing.
"A few hours later, Clarke was looking at a blueprint for an 8,000-pound machine that used hydraulics and electricity to focus increasing amounts of pressure and heat on the core of a sphere. The device, he was told, re-created the conditions 100 miles below Earth's surface, where diamonds form. Put a sliver of a diamond in the core, inject some carbon, and voil�, a larger diamond will grow around the sliver."
https://www.gia.edu/sites/Satellite?c=Page&childpagename=GIA...
The IIDGR [1] company makes a series of devices (DiamondSure, DiamondView and DiamondPlus) that will indicate whether a diamond is a natural (and if so, of what type), or flag it for further characterization. One system will flag type IIa naturals for further analysis, as they resemble the best of CVD synthetic diamonds. Microscopy + (IR, fluorescence, and photoluminescence) spectroscopy distinguish a stone's origin to high confidence. Even though the best CVD stones have far lower impurities than the best naturals, they have internal strain patterns (optical birefringence) that can be diagnostic.
Most jewelers don't have these machines, but they are available as a service.
Faceted CVD stones have been seen up to 5 carats [2] and 10 carat stones made by HPHT are known [3]
[1] https://www.iidgr.com [2] https://www.gia.edu/gems-gemology/winter-2016-labnotes-CVD-s... [3] https://www.gia.edu/gems-gemology/winter-2016-labnotes-blue-...
It's sort of funny in a way... impure diamonds are worth more than pure diamonds... but then again, none of them would be worth much at all if DeBeers didn't control the release into the market (diamonds are not rare at all, and therefore only valuable due to artificial scarcity).
This was a while ago though so I'd imagine the cat & mouse game has evolved for both sides.
The testers that can identify a diamond's origin are quite expensive and usually only found at major gemological labs.
All synthetic white diamonds are "Type IIa" (no or trace amounts of nitrogen) while only 1-2% of mined diamonds are IIa. 97% of mined diamonds are "Type Ia" (clusters of nitrogen). It is possible that is what they were referring to about purity, but the amount of nitrogen is not a definitive indicator of origin.