Oh good. I don't have to be surprised any more.
I've only skimmed the Shibowhatsa of Feanor once, and only read carefully on bits and pieces of it. I have to wait until my summer classes are done before reading it.
I guess Maglor didn't have kids then?
And Diamond, I just thought of something, if you took jewels and got them really warm and then stuck them inside a blanket, then jewels could help you keep warm.
And actually, Maehdros's Silmaril helped him keep his hands warm for a while, and then it caused him to go someplace even warmer.
So I'd say a jewel can do anything that a blanket can do (and jewels are prettier).
Or were you not hinting at blankets when you said jewels couldn't keep you warm at night?