Hippocrates and Galen advanced the concept of humorism, a theory which held that we get sick from imbalances of the four basic substances within the human body, which they identified as blood, phlegm, black bile and yellow bile.
If I bought a pair of $190,000 Focal Grande Utopia EM and found out the beryllium isn't any more pure than a pair of $2700 speakers, I'd be very upset. I would also be upset if the beryllium tweeters in my $190K speakers cost less than diamond tweeters in a pair of $5K speakers.
Diamond may be considered the best hard dome tweeter by B&W. It is certainly the most expensive. The cheapest aftermarket diamond tweeter is $2,000 each. A pair of diamond tweeter speakers would cost $4,000 for just the tweeters alone. Dang, the B&W 805D2 seems like a bargain at only $4,000 delivered from some dealers ($5K MSRP). But others from Revel, Focal, Paradigm, RBH, etc, believe that Be is the best driver period.