Stereophile has an interesting paragraph on the Magico Be-Diamond tweeter:
"A beryllium dome is both light enough and stiff enough to behave pistonically, and was used in the Magico Q5, which Michael Fremer reviewed in November 2012. Applying a layer of diamond to the metal, Tammam explained, results in a dome with a more homogeneous surface, which both reduces intermodulation distortion and results in a more benign harmonic-distortion signature that is less like that of a metal dome. I asked why they hadn't gone all the way and used an all-diamond diaphragm. It turned out that, yes, diamond would produce a very stiff diaphragm, but the required suspension would raise the tweeter's low-frequency resonance from the desired 500Hz or so to about 1.3kHz. This, in turn, would mean that the tweeter would have to be crossed over to the midrange drive-unit at too high a frequency. Beryllium's lower mass ensures that the resonance frequency is close to 500Hz, but the diamond layer raises the dome's stiffness to extend the high frequencies."
"A beryllium dome is both light enough and stiff enough to behave pistonically, and was used in the Magico Q5, which Michael Fremer reviewed in November 2012. Applying a layer of diamond to the metal, Tammam explained, results in a dome with a more homogeneous surface, which both reduces intermodulation distortion and results in a more benign harmonic-distortion signature that is less like that of a metal dome. I asked why they hadn't gone all the way and used an all-diamond diaphragm. It turned out that, yes, diamond would produce a very stiff diaphragm, but the required suspension would raise the tweeter's low-frequency resonance from the desired 500Hz or so to about 1.3kHz. This, in turn, would mean that the tweeter would have to be crossed over to the midrange drive-unit at too high a frequency. Beryllium's lower mass ensures that the resonance frequency is close to 500Hz, but the diamond layer raises the dome's stiffness to extend the high frequencies."