Questonite

Gram for gram this is by far the hardest, toughest, most resilient material known to man. In fact, it may well be the strongest workable material in the Galaxy, since past a certain point most technological civilizations switch to pure energy force-fields of one sort or another.

It can only be worked for a brief period after its initial creation; after that, no known tools will so much as scratch it.

In addition to being physically tough, Questonite has a number of energy-disrupting properties that make it extremely difficult to phase through, or teleport through, or otherwise bypass. This would make it the ideal protective material if it weren't so incredibly expensive to make.

The formula for making it is one of the world's most restricted technological secrets.