@musicaddict - You are right that none of them are "natural" diamonds. I suspect some of the most famous brands with diamond tweeters may be coated with diamond dust from natural diamonds like the common nail file.
The most ridiculous use of actual single crystal diamonds is probably from the likes of Accuton and the Chemical Vapor Deposition they use for their mids and tweeters.
I’m afraid Accuton for me falls into the twin categories of amazing engineering and not really interesting to listen to.