Ok, so i've seen alot of talk about Diadems around here. I understand they have a Magic Level, which affects the possible mods, affixes, or something.
Now, I found a three socket ethereal one. This seemed like it could be awesome. Or maybe I don't understand this yet. Would this be good for an imbue? Or for a runeword? Or just put gems/jewels in it for a merc or MF?
This is not something to be worth anything. Usually rare circlet/coronet/tiaras/diadems can be godly (or some magic ones).
Diadem has highest magic_lvl, so each diadem can get every affix, that's why they are usually used when rerolled ilvl does decrease with 6pskull rare diadem reroll, but it still can gain all the affixes.
I'm nitpicking a bit here, but a diadem can get any affix because it has a very high base qlvl (85). The magic_lvl is still essential to make it undergo a more advantageous affix level calculation procedure, but a magic_lvl of 5 would be enough for getting any affix while it actually has one of 18.