Niddah 22 - Hair
If a woman expels objects resembling a rind, a hair, dust, or midges, all of red color, there is not question that these items are not embryos or fetuses. Therefore she is definitely not impure with the impurity of childbirth. However, these items may be coagulated menstrual blood, and then she would be impure with the regular impurity of a niddah .
In that case, she should test them by placing them in water and soaking them there. If they dissolve, this is menstrual blood, and she is impure, but if not, she is pure. In the previous case of a formless mass , the teacher did not prescribe this test, because he is of the opinion that menstrual blood does not coagulate into a large mass, but only into a small object the size of midge or hair.
Art: William-Adolphe Bouguereau - Italian Girl Drawing Water