Niddah 60 - Woman Borrows a Tunic
If a Jewish woman lent her tunic to a gentile woman, or if she lent it to a Jewess who was a niddah at this time, and when she got the tunic back and wore it herself, she afterwards discovered a bloodstain on it, she may attribute this stain to that other woman she lent it to. The other woman does not loose anything thereby: if she already was a niddah, this does not change her status, and if she was a gentile, who does not observe these laws, the Sages presumed that she is a niddah at all times.
However, if three women who were ritually pure wore a single tunic one after another, and afterwards blood was found on it, none of them can blame the other with certainty, and not being able to shift the possible responsibility, they all three becomes ritually impure.
If they sat on a stone bench, though, they are all ritually pure, because, as we saw earlier , any object that does not accept ritual impurity, does not affect the stains either. Rabbi Nechemyah formulated a rule: "Anything that is not susceptible to ritual impurity is not susceptible to stains." His general rule includes even such cases as sitting on the outside of a clay vessels, since clay vessels do not receive their ritual impurity by contact with their outside surface.
Art: By (after) Dyck, Sir Anthony van - Portrait of two sisters, one in a blue dress, the other in a brown dress holding a bouquet of flowers