An Iron Nugget is a craftable, renewable, non-placeable item. It has a texture resembling that of the Gold Nugget, recolored and turned 90° to the left, to prevent mistaking the item as a Ghast Tear. Its ID is
There are only two methods to obtain Iron Nuggets:
Crafting an ingot to get 9 nuggets.
Smelting any iron tool or any iron or chain armor, including horse armor, obtains a single nugget.
There are four other recipes involving Iron Nuggets as a component: Iron Ingots, Chains, Lanterns and Soul Lanterns:
Image source: Minecraft Wiki