Orichalcum Ore

Breaking altars decides which hardmode ores you get, and the second tier gives you either this or its counterpart. You do not pick. Smashing more altars only produces more of what the world already settled on, so if you want the other metal the answer is a second world or a friend, not another swing of the hammer.

Nothing from before hardmode will scratch it. You have to mine and smelt the tier below first and make a pickaxe out of it before this becomes anything more than a tease on the screen. That two step gate is most of why the first hardmode mining trip feels so slow, and explosives are not a shortcut, since the ore shrugs off nearly everything you can detonate.

Hardmode crates draw from the same pool, so a rod quietly builds a stockpile while you do something else. On a world where you broke a lot of altars and now have wraiths patrolling every cave, that is often the calmer way to finish an armor set.

Orichalcum is a metal Plato wrote about as the wealth of Atlantis, and nobody has ever identified what it actually was. Ancient descriptions point at some kind of copper alloy, which would put it closer to red than the magenta it has here. The magenta keeps it visually distinct from the other two metals at the same point in the game.