משנה על שבת 6:1

Any object that is susceptible to corpse uncleanness, though not susceptible to midras uncleanness, is still susceptible to negaim uncleanness. For instance: the sail of a ship, a curtain, the forehead band of a hair-net, the wrappings of scrolls, a coiled belt, the straps of a shoe or sandal that are at least as wide as a split bean, Behold these are susceptible to the uncleanness of negaim. A thick cloak on which a nega appeared: Rabbi Eliezer ben Jacob says: unless it appears on the texture and on the stuffing. A skin bottle or a shepherd's leather wallet are inspected in the position in which they are used, and a nega may effectively spread from its inner side to its outer side and from its outer side to its inner side.

Wood that serves a metal vessel is susceptible to impurity, but metal that serves a wooden vessel is clean. How so? If a lock is of wood and its clutches are of metal, even if only one of them is so, it is susceptible to impurity, but if the lock is of metal and its clutches are of wood, it is clean. If a ring was of metal and its seal of coral, it is susceptible to impurity, but if the ring was of coral and its seal of metal, it is clean. The tooth in the plate of a lock or in a key is susceptible to impurity by itself.