I heard the $4000 Focal Utopia Headphones yesterday

I belive beryllium is dangerous during the manufacturing process but left alone is stable.  It is brittle and can be poked with a finger which is  why there is a metal grill over the beryllium mid/tweeter of my TAD's.   

Beryllium is toxic, period. Skin contact usually only raises a rash, but forming the stuff is always left to specialty shops, which is why you can't just buy some to play with. Beryllium copper alloy is mostly copper, which is why it is safe to use.

Don't know if they are worth a try at $4000 but I have a pair of Shure SE846 IEM for $1500 and I have never heard anything even remotely as good in an IEM.  These IEM sound very close to the very best open back cans I have heard and have incredible noise reduction because of the great seal with memory foam.

Beryllium dust is deadly but as no one will be subject to beryllium dust from a set of headphones I think you are good. I don't suggest that anyone should extract the beryllium diaphragm, drill it to create a cloud of dust and then inhale deeply :) other than that you are safe.

Who'd thought we've come to the age of weaponized drivers?