One problem that has pagued nearly all of the forts that I have ever made... all this #$%@in' rock everywhere!
So, I propose a use to it, drag it to the smelter and attempt to extract trace metals from it. The process of extracting would involve looking at the raws and determining the naturally occuring metal types found in that type of rock and then giving 10% (maybe less... 5%, 1%?) of a bar of that metal and destroying said rock. Still uses the same (although slightly more) dwarf-hours for cleaning up stone, but very useful compaired to a quantum stockpile that you will never use.
Naturally, this process would require fuel, so working at a normal furnace would require 1 unit of fuel per rock smelted and just magma at a magma furnace. Still uses the furnace opperator job and should be the same time-per-skill as melting normal ore.
Designating should be similar to marking things for dumping, but not the same as melt down metal item. The reason for this, is that I would like these two actions (melt item and melt rock) to be different when jobbing a furnace, so when my little dudes are melting down rocks for little scraps of metal, but a goblin invasion happens and I end up with a bunch of goblin weapons/armour, I can disable the melt rock job on the furnaces and prioritise melting down the goblin stuff.
Super mode: Allow us to chuck a dead goblin (armour and all) into the furnace and attempt to melt the metal out of it!
Even more super mode: Magma furnace trap... goblin triggers a tripwire and gets dropped into said furnace... out comes screams of agnony followed by metal bars!
(just don't forget to crank up the heat so they don't stink as bad!)