What are the differences in Tweeter materials?

Getting back to the OP, I think most people break it down to metal domes, fabric domes or ribbons. There are others exotic ones, such as plasma, but that's not really the issue, IMO. Many people seem to think that metal domes are harsh and silk domes are sweet - after all, it's silk, right? From what I have heard in many years of involvement in this hobby, is that a speaker designer can manipulate the output of a tweeter by padding it down or mounting it in a certain way. So it's more the designer's choices than the tweeter material that will determine the sound. Remember - bright speakers are initially impressive, leading to a quick sale. However, this becomes tiresome at home - hence the endless posts here about bright or harsh systems. The moral is - listen to the speaker as a whole and use your ears. The tweeter or woofer or cabinet material might be intellectually interesting, but ultimately, you have to like listening to the thing.