癸 is a stem, not a substance. It is the tenth of the heavenly stems and the one paired with water in the five-phase scheme, so 天癸 is something like “the heavenly water” — an endowment named by its place in a calendar rather than by what it looks like.

The chapter then puts it on a timetable: it arrives at two sevens and is exhausted at seven sevens. That schedule, rather than any moral failing, is what the chapter gives as the reason fertility ends.