With a full mana pool, throw 2 manasteel, 2 mana perls, and 2 mana diamonds on the beacon. after a few seconds, 1/2 of the mana pool will be used, as well as all of the resources initially dropped on the beacon, however, only 1 terrasteel is produced.

MC1.7
Forge 1085
Beta-37

I'll have to add a check for stack sizes, you should only be able to craft one at once.