is a specific kind of: natural object.

is a broader term that covers: scale | shell | test | body covering | case, sheath | integument | blanket, mantle | crust, encrustation, incrustation | envelope | cap, hood | indument, indumentum | roof | shell | eggshell, shell | skim | slough | snowcap | vesture | peridium | pericarp, seed vessel | chlamys, floral envelope, perianth, perigone, perigonium | sac, theca | indusium | bark ... show more.

derivation: cover, spread over.