Why did Moshe Rabbeinu balk when the women wanted to donate their mirrors to the construction of the Mishkan (Rashi, Shemot 38:8), but not when the women donated their kumaz (Shemot 35:22), which were a piece of jewelry worn in the nether regions of the body (Rashi)?
Ramban on this verse says that the difference is that the gold from the kumazes was mixed with the rest of the gold, whereas the kiyor was made only from these mirrors, without any admixture of anything else.
Moshav Zekeinim (a collection of commentaries from the schools of Tosafos) takes a slightly different tack: the kumazes were melted down and so were no longer identifiable, whereas the mirrors were affixed to the kiyor as they were (perhaps, although they don't say so, somewhat like the fire pans used to plate the altar, Num. 17:3-4).
I understood it as a chastity belt of sorts. This would seem (IMHO) to be the exact opposite purpose of what the mirrors were used for (mirrors strengthen the Yetzer HaRa', in contradistinction to the purpose of the Kumaz, which is to fight off the Yetzer HaRa').
Similarly, according to this related essay in What's Bothering Rashi, RaMBa"N discusses the subject and states that the mirrors were rejected because they are "used for the Yetzer HaRa". (Unfortunately I just can't find it in the RaMBa"N; the cited source is incorrect.)
A quick, cursory Google search turns up some interesting questions about the strangeness of it and the conjugation of the verse, but very little substance about what it actually is.