Titanium Ore

Breaking altars seeds the third tier of hardmode ore into your world, and the game decides whether that tier is this one or Adamantite. A single world will not give you both from altars, so if you want the other you are fishing crates or trading with a second world. Every further altar you smash brings in less than the one before, and each one also raises the amount of corruption or crimson in the world, so break a couple, go mine, and only break more when you actually run short.

The armor and tools are useful but replaceable. The hardmode forge is not. Everything above this tier needs one to smelt, so the first priority on a fresh vein is getting enough for that forge rather than a full set of gear. Do that and the rest of hardmode opens up.

It sits down at lava depth and shrugs off ordinary explosives, so the bomb tricks that make earlier ore runs fast do not work here. It also needs a pickaxe from the previous hardmode tier, which means the ore chain has to be walked in order. Expect a proper lava-proofed trip rather than a quick dig.

The real metal is a plain silvery white, and the purple sheen on the in-game ore is invented. Titanium is one of only three hardmode ores named after a metal that actually exists.