Are my " diamond " tweeters really diamond

The idea of using synthetic diamond is to use an extremely stiff material so that the resonance/breakup frequency is raised up higher than human hearing. I believe the breakup frequency of diamond is around 70khz. I think beryllium could be around 40-45khz. With metal dome tweeters, the breakup is very low, sometimes as low as 22khz. This is why metal dome can sound harsh/bright.

Which is irrelevant, because no one (especially the average 50+ year old audiophile), can hear above 20 kHz. This is why many flagship speakers, some costing 10s of thousands still use basic aluminum domes. 

^ It's marketing, pure and simple.

I have never heard any improvement in sound. I much prefer a nice soft dome tweeter. I may be old but those tweets that go way up in  the frequency range make me edgy and nervous. I think it is the distortion or break up at " inaudible frequencies " really can be perceived.

I've heard a few soft domes that sound bright and edgy (for example, some from Polk) and metal domes that are smoother than a typical silk dome. Some high-end soft domes break up under 20 kHz. Why do they not sound edgy? Because it's all in the quality of the parts and implementation. I think many mistake upper midrange distortion as tweeter breakup.

@luizfcoimbria,

Sure,  there have been advancements in some areas. IMO, the greatest gains have been in small, budget speaker performance. It's easy to make a large, heavy, cost-no-object speaker sound great. It doesn't have to push the boundaries of physics. 

The best speakers I've heard to date don't use any esoteric materials or cabinets. In fact, they're simply updates of 30+ year old designs. For example, Stirling Broadcast's LS3/6. They use a plastic dome super tweeter with a published frequency roll-off point of 17kHz. Many audiophiles would see that number and immediately dismiss the speaker as inferior. They'd assume these speakers lack detail or "air" compared to those equipped with a diamond or beryllium tweeter. Yet, IME, these speakers have greater resolution and detail than the B&W Diamond series and other highly regarded brands.