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.