There are two sorts of necessity: in itself and from another. Necessity is simply what cannot not be, but a thing can have this either as a fact or by logical necessity. The “cannot” in the definition can either be physical or logical.
We know of substances so hard that they cannot be scratched by anything other, it is not much of a jump to substances so solid they cannot be dissolved or corrupted by any other. Such substances would be necessary, but, for all that, we might make them in a lab. Matter seems to have this sort of necessity, though not from its rigidity but from its ability to remain throughout changes.
Now physical necessity suffices for an explanation of the contingent by the necessary, and if we are right that all necessity is physical or logical, and that only logical necessity is a necessity from itself, then physical necessity is from another.