I have a fascinating book "The New Alchemists" talking about the early history of manufacturing synthetic diamonds. The book said that back in the 50s or 60s—don't remember which—it took some multi-million dollar facility to make diamonds—but back when the book was written (late 80s?) they said that anyone with a rather large basement and $50 000 to spare could create diamonds.
Historically most synthetic diamonds were small low grade industrial abrasive-sized diamonds. Gem sized diamonds can be produced but you spend much more time cooking a given batch. At the time, only the Japanese seem to have spent much effort on creating large gem quality diamonds.
A decade or more after the book was written the Soviet Union broke up and creating gem quality diamonds became a popular cottage industry in parts of Russia.
I'm a decade or two out of date but it has become increasingly difficult to distinguish between man made and real diamonds. I can't see that it matters.
Last account I had—man made diamonds have microscopic inclusions of Aluminum that can only be detected with X-Ray crystallography.
Check out Moissanite—gem quality Calcium Carbide. It is almost as hard as Diamond. It has a greater index of refraction and while it isn't dirt cheap, it is noticeably cheaper carat per carat than diamonds. I'd rather have a 2-carat moissanite than a -.5 carat diamond—or whatever the equivalent price would be.