The rock melter uses frictional heating to melt rock into lava, which can be drawn out with pipes. Netherrack provides the most lava per block, while cobblestone provides the least.
To melt cobblestone, you need to get the melter up to 1000C. This will require significantly more power than a single steam engine can produce.
Netherrack, however, melts far more readily, and a single steam engine will easily melt netherrack into liquid lava.
Power Input: Bottom