Δαφνη Ευτυχια Αρθουρος

(Awake, but not really "here" 'cause I have to run out and take care of an errand. Wanted to squirt this onto the wires before I had time for it to slip my mind. More of that dangerous shower-thinking, if you're curious.)

I'm looking for a substance that would be incredibly difficult to penetrate mechanically but transparent to electrommagnetic radiation at nearly all frequencies (at least all non-visible ones). Something about which clueful folks might intelligently say, "We have no way to cut, drill, or break that." My first thought, of course, was diamond (this was originally going to be an entry asking whether diamond is radio-transparent), but hey, people cut diamonds all the time by fracturing them, so what would stop someone from smashing a diamond box by hitting it hard enough to break it along planes of the crystal?

Ideally, I want something non-metallic (see note about transparency to non-visible EM), which rules out adamantium, and besides, I'd rather a) use a real-world substance if one exists and b) avoid swiping someone else's fictional substance unless it really makes things a lot easier on the reader (assuming I ever get around to writing the story-idea that came to me in the shower). [Edit: I've just rememberd that General Products hull material would work, except that my comments about preferring a real-world substance and being hesitant to use someone else's fictional substance still stand.] I've already got an idea for what to do if I have to make something up with a bit of technobabble. Expense is not a factor; I'll just assume that the person building an enclosure out of this material is incredibly wealthy. And possibly very patient. The box made of this stuff will be a cubic meter or so, and able to survive having the building it's in catch fire and collapse.

Transparency in the 1GHz-100GHz range is required; RF below 1GHz would be useful, as would 60Hz, but if it's opaque or severely attenuates the lower frequencies I can work around that. Opacity at near-IR, visible, and higher frequencies is desirable but not required. And I know so little about materials-science that I'm not even sure where to start to construct a Google search string. Anyone know what I'm looking for, or should I go tune my technobabble?