[baybal2]

Vacuum dielectric strength is 20 megavolts per metre.

Diamond is almost perfect though, at 20 gigavolts per metre, and that only given if the diamond has at least some microscopic imperfections.

[analognoise]

I think those might be reversed...

[blattimwind]

IIRC diamonds are good isolators because their crystal structure is both very close to perfect (few defects) and because their crystal lattice has all electrons tied up.