Your lucky they are Roor parts or you likely would have broken them in an atempt to seperate them, I once had a stem stuck in a bong, I used non toxic kitchen lube that I got from the gas station that I used to work at, it worked like a charm, the trick is getting the lube in between the two parts, which I can see is your problem. I cant think of anything else other than trying to put some pressure on it to loosen it up, just be careful it being glass and all!
Good Luck