The Heart of Lorkhan is listed as the original stone that helped creat the Red Mountian Tower and the orignal stone in the Walk-Brass Tower. Was the HEart taken from Red Mountain to aid in the construction of the Brass Tower?
Also, so long as we're talking about it, if the Eye of Magnus is a stone, to what tower would it belong? It's possible that it's the Throat of the World, but many seem to believe that the stone there is Alduin (ar at least a Dov of some sort). My believe on the Throat is that it's either Dragons in general, or the Thu'um. In which case, so long as the Greybeards are there, it stays active.
Now one final thing to make it all a little more interesting. We call the Towers listed the 'Known" towers. Who knows what things have been lost to time and the ages? I believe there are more Towers than we currently know about, possibly on other continents, but I think there are others a little closer to home. One vague and fleating possibility in my mind is that of the Skyforge. It's an unknown bit of history, and was already there when Ysgramor(sp?) came about the area. What are the thoughts on this, and what are some other possible Towers?