I did a fair amount of research on this when shopping for my wife, and couldn't find any decent sized (say, larger than ~1/2 carat) clear synthetic diamonds. There were some options in yellow or pink, but I didn't find a thriving manufactured diamond industry I thought I would find.

Regardless, even if I had, manufacturers of gem grade diamonds wouldn't need to undercut the price of natural diamonds by much. The cartel has done all the hard work raising the price for them already. Market them at 90% of natural diamonds and reap the rewards.