Kai: Maybe your thinking of Zesuanium coated armor?
Soresu: You're right. Given that it's totally immune to all conventional forces (heat, magnetism, electricity, kinetic energy (bending, shattering)), it must be of an incredible magnitude of density. Like degenerate matter ("neutronium"). Realistically such a material would likely weigh hundreds of thousands (!) of pounds per teaspoon, but for the sake of convenience in roleplay it seems to be less. It should certainly still be a lot heavier than gold though, by several magnitudes, if not douzens.