Iron Mining Cells can only be crafted after you have unlocked their crafting recipe by taking/crafting an Obsidian Mining Cell.
To craft one Iron Mining Cell, you'll need (as of R43 in May 2017):
- 4 Iron Bars made in a Forge from Iron Ore from the Stalactite layer or found in Iron Treasure Chests spawning on Lava layer-blocks
- 2 Obsidian Slabs made of Obsidian (bars) in a Processor
- 8 (blocks of) Saltrock mined from the Stalactite layer, which requires at least an Obsidian Mining Cell
- 8 (blocks of) Glowing Mushrooms picked from the Stalactite layer, which requires at least an Obsidian Mining Cell or looted/pet-harvested from Mirus or found in Diamond Treasure Chests
- 4 Seashells found on Sand near Water at any lake or Ocean Shore, made from Sand with a Plow, or looted/pet-harvested from Pebbles
However you cannot pull even harder rocks with Iron Mining Cells - like any corrupted blocks from the Corruption layer.
Taking/crafting Iron Mining Cells is one of the requirements to unlock the crafting recipes for Diamond Mining Cells, Freeze Bombs, Freeze Resistance Potions, Fire Resistance Potions and (basic) Excavators, (since update R34).
Since update R29, one Iron Mining Cell can mine up to 350 blocks as hard as those of the Lava layer (T4) before it will fall apart completely - or up to 700 blocks as hard as those of the Stalactite layer and/or Fossil layer (T3/T2), plus it can mine infinite amounts of blocks as hard as Stone (T1) or organic matter like Dirt, Wood and the like (T0).