What are the differences in Tweeter materials?

Well, I'll chime in, I've built with several tweeters and can speak from experience about some of their general characteristics... remember, no absolutes here, if you change the dampening material on any dome, the dome's sound changes. Let me preface this by also saying that I have never personally paid more than $100 or so for a dome, so my experience is limited to that, yet I have personally used dozens upon dozens of tweeters.
In general, the harder the material, the higher the material rings. You will see many metal domes that have a rise and some have a break up very high. In general, the harder materials have a more detailed sound, which is why you've heard that many hard materials can sound harsh. If a hard dome, whether diamond, ceramic or metal is properly damped, it should be very detailed and today, well damped hard domes are capable of some of the most detailed sound while remaining smooth with the right gear and front end. A great 3/4 inch metal dome is Hiquphon.
My least favorite domes have been mylar domes, cone breakup is very audible and these cones can't be pushed. Treated fabric has been mastered. I particulary like some of the better Seas and a Peerless HD (now scan speak), but many companies make a great fabric dome. While a fabric dome typically is not as detailed as a metal in the last degree, they are none the less, detailed, have an extended high end and offer low resonance so they can be crossed over with a variety of drivers.
Ribbons, are very fast & smooth sounding, but typically have a large variance in response curve and unless you use notch filters, most ribbons have peaks that I have found undesirable. Most ribbons also have a higher resonance hence they cannot be crossed very low. I have no experience with the mega buck ribbons and yes, they seem to have curves that affordable ribbons can't match.
Remember, these are very general statements and anyone with real experience could find a single driver to challenge a statement here. I hope this helps, Tim

@Timlub,

Thanks for your post. My favorite soft domes have been from Dynaudio, Morel, and ScanSpeak. I find a lot of todays Seas soft dome tweeters to sound a bit on the bright side. I was not happy when Spendor switched from ScanSpeak to Seas for their tweeters. Especially in their Classic line. They always used ScanSpeak. I have a pair of Proacs that also uses the 3/4 inch ScanSpeak. Great tweeter. Wonder if they are still in business or did they get bought out by Peerless?

Mezzanine,

I'm glad you posted. My comment about the secret pricing structure, I hope, was taken just as some light hearted humour, and nothing more. Someone always seems to be complaining and/or joking about the high cost of some equipment, I just couldn't resist.

Unless I misread your post, it looked like you were trying to compare some of the more common materials to some of the more rare/exotic ones, with an emphasis on soft tweeters. API (Audio Products International), the parent company of several Canadian high end speaker manufacturers (Mirage, Energy, Paradigm, PSB and Athena are the ones I can remember. There may be others as well), make camel hair tweeters. They put them in Energy speakers and I think the original Mirage M series. I know several people that have these tweeters and swear by them. I'm not sure if they are currently going into any speakers but I'm pretty sure you can still buy the raw drivers.

Mezzanine, the Dyn and ScanSpeak tweeters are both very good, the few Morels that I have used had very good response curves and used at the right frequencies are excellent, not as detailed as they could be, but I have also found the Morels to have high HD that is very audible when crossed low.

What about magnésium. And the musicality..?