Iron Ore

When iron ore is mined, you get iron ore rather than Iron Ingot but when you smelt the ore after you mine it, you then get the iron ingot. Iron ore is uncommon but not rare. Iron ore is found underground so a abandoned cave system will most likely lead you to iron ore.

Iron ore is needed in order to complete the achievement "The end" because it is needed for a iron pickaxe to mine diamonds. After the diamonds are mined, it gets turned into a pickaxe for obsidian. The portal is made and the Eye of ender gets thrown.