Terrasteel

Botania
Terrasteel is created via an infusion ritual atop a Terrestrial Agglomeration Plate. To form one Terrasteel Ingot, throw a Manasteel Ingot, a Mana Pearl, and a Mana Diamond onto the plate. The plate then must receive 500,000 Mana (Sparks are the best way to do this), without any of the 3 items being picked up. If any of the items are picked up, the infusion will fail. As an indication of progress, several wisps of glowing blue smoke will fly out from the plate, rotate, fade from blue to green, and finally crash together on the items, combining them into Terrasteel.

Note that the Manasteel ingot, Mana Pearl and Mana Diamond are treated as thrown items and thus would despawn after 5 minutes. Therefore, it is important to make sure you have enough mana to finish the ritual lest you lose your progress and items.

Flaxbeard's Steam Power
Flaxbeard's Steam Power only adds the plate variation of the Terrasteel material. The Terrasteel Plate can be created by throwing Terrasteel Ingots into a heated Crucible, and then pouring that molten metal into a Mold with a Plate Mold. Six nuggets are used in the creation of a Plate, which makes it unable to be used in other mod's recipes requiring their own Terrasteel Plates.