Platonium
This stuff can only be made in a particle accelerator that has been consecrated to the worship of Hephaestus by a priestess of the Cabeiri mystery cult. Fortunately, there are any number of engineering fraternities that can provide the necessary personnel, which is one reason why the American Greek system continues to exist on the collegiate level. The esoteric benefits are simply still too vital for national security.
Anyway, Platonium is expensive to make, but worth it. It’s a water-soluble nontoxic green metal that is somehow more real than everything else in the universe. Alloys incorporating the stuff simply don’t take more than cosmetic damage from non-Platonium enhanced sources. Clothes colors with Platonium-saturated dyes don’t tear, rip, or stain (this includes things like tar). Drink a glass of Platonium water (tastes awful, by the way), and your next bullet to the gut becomes a bloody flesh wound. The only drawback to Platonium is that it will not permanently mix with anything: the general rule of thumb is that it will completely disappear from an alloy or mixture after a month, and exit living tissue within a day of ingestion. There are no ill effects to living tissue, but when a nonliving, unhealing item finally loses all of its Platonium, the results can be spectacular.
In case it isn’t obvious at this point: Platonium production is reserved for nation-states, international organizations, and the more reputable sorts of corporations. If anybody else starts making the stuff then the appropriate agency typically sends in somebody hopped up to the gills on Platonium in order to disrupt production. If that doesn’t work – and it usually does – then the same team typically sends in a team, to do the job more subtly…