An encrustation is a hard layer that has formed over a surface — as if a crust had grown there and stuck. You will hear it for salt on sea rocks, minerals inside old pipes, rust, barnacles on a ship, or, in a prettier stretch, jewels set thickly on metal. The tidy gloss “covered with a hard layer” names the state; this word is a noun for that layer, or for the process of it forming. It is fairly formal or scientific. The everyday verb is encrust; the adjective is encrusted. Bread crust shares the picture of a hard outside, but that is food, not what people mean here.