Made of Indestructium: … alas, the universe is hard on indestructium.
About as close as nature gets is probably neutronium – and whatever even more degenerate forms of quark matter, etc., you can get beyond it. Sadly for engineers everywhere, neutronium is rather hard to work at the best of times, behaving essentially like a fluid, and having a really nasty habit of evaporating in a giant whuff of neutron radiation the moment you remove it from the deep, deep gravity well necessary to make the stuff. Metastable neutronium would be nice, and there are people working on that…
In somewhat more practical terms, muon metals, which is what you get when you strip all the electrons out of metal and replace them with muons, their leptonic cousins. Since muons have the same charge as the electron but greater mass, they have much smaller ground-state waveforms than electrons in the atoms thus formed, resulting in matter than has similar chemistry – albeit rather more endothermic – to the original, but whose density and physical properties in re energy-resistance are pushed way, way, way up as the atomic spacing shrinks way down. It would make good armor, if the mass penalty wasn’t, inevitably, quite so harsh. On the other hand, it’s one of the things that makes torch drives practical (being so incredibly refractory, and thus letting you push the drive output/waste heat/resulting radiation rather further than you otherwise could), and also is invaluable to coat lighthugger wake shields with, being able to easily shrug off the sort of dust-particle impacts you get when plowing through interstellar space at 0.9c.
But neither of these is actual indestructium, ’cause, well, antimatter. Neutronium and antineutronium will annihilate quite nicely, and while regular antimatter isn’t quite as corrosive to muon matter as it is to everything else – an antimuon is not a positron – the proton-antiproton annihilation will proceed as normal and will make the whole thing come apart just fine.
Alas, indestructium, we barely knew ye.
(There’s also singularity-locking, the handwavium I promised to explain last time. That’s actually a simple reuse of existing handwavium – vector control – in this case being used to grab and redirect, while conserving, the momentum of things that would otherwise impact the surface of the singularity-locked thing into a giant kinetic energy sink.
The reason it’s called singularity-locking is because the sort of giant kinetic energy sink you want for this is a modestly-sized black hole. This is why stargates use it, because they already have a modestly-sized entangled kernel sitting in there to make their primary function work, so you might as well get the extra use out of it. It’s also why nothing else does, because if you think muon metals have a harsh mass penalty, they’ve got nothing on dragging millions of tons of hole around with you to make your armor work. A mass ratio of what, again?
[Also, people – with fairly good reason – don’t exactly want one in their back yard anyway, on general principles.]
Sadly, this isn’t pure-quill indestructium either, technically – while it would require a ridiculous amount of energy, it is theoretically possible to overload either the singularity-locking systems or the K-sink itself, and boom. Fortunately, it would be so much boom that so far no-one’s seemed inclined to hit a stargate with a small moon and see what happens…)