Abstract: A primarily polycrystalline but partially amorphous electrical insulator can hermetically seal first and second spaced electrical terminals, one made from an anodized aluminum and the second made from a beryllium copper, Kovar, an alloy of iron and cobalt or an alloy of beryllium, copper, nickel and gold. Nickel may be diffused into the beryllium copper and a noble metal may be deposited on the nickel. The insulator provides a flat meniscus to abut a corresponding electrical insulator in a cable. The insulator may provide an electrical impedance of approximately 50 ohms, an electrical resistivity greater than approximately 10.sup.18 ohms and a dielectric constant of approximately 6.3. The insulator operates satisfactorily in a frequency range to approximately 40 gigahertz.