The N28-BER by Transducer Labs that Fritz uses in his Carrera speaker is the only one I've heard that I think I could live with. BTW, TL describes this on their website as pure beryllium.
beryllium vs diamond
Hi guys, today's technology has brought us a new type of tweeter made of diamond or beryllium. Do you know what are the strengths and weaknesses of diamond vs beryllium? Which one is the more expensive? Has today's dome tweeter better resolving power than the venerable electrostat? Jim Thiel once said that dynamic designs will be getting better all the time and will probably surpass electrostatic designs.