I would like to comment on the response to Beryllium as respiratory toxin. Beryliosis is caused by inhalation of the beryllium particles into the airway. In order for beryllium to go into aerosol form, you have to essentially have to heat the tweeter to it's melting point. Since the melting point of heavy metals are fairly high this would not usually in normal operating conditions of a speaker. Beryllium is toxic, however. It would not be a good idea to lick the tweeter and put yourself at risk for beryllium toxicity. Fortunately, in my 11 year career as pulmonologist, I have yet to see a case of Berylliosis.
Kelvin Panesar