Not a stupid question at all....

Sounds like liquid iron to me!

Actually - if I am not mistaken (help me here Alan!)- it actually IS a type of very fine powdered iron suspended in some sort of oil or silicone that is injected around the voice coil of (usually) tweeters and thus it "sticks" to the magnets. It is supposed to help with disipating heat from the voice coil (cooling and such) but not restrict movement.

I know that AR used to make tweeters with this (and perhaps they still do) and I think that there are other companies that do it as well.

Randyman