Demonite Ore

Natural veins exist but they are scarce and you will spend more time looking than mining. The Eater of Worlds is the real supply line: it comes apart into ore every time you fight it, and the fight is cheap to restart once you can make Worm Food. If your world rolled Corruption, the Eye of Cthulhu adds a smaller pile. Ore you do not need still sells well, so a repeat farm pays either way.

Deep veins demand a better pickaxe than the ore itself lets you build, the usual chicken and egg. Explosives sidestep the whole thing. Bombs from the Demolitionist knock the ore loose no matter what you are holding, and a couple of stacks turns into a full set far ahead of schedule.

Unlike nearly everything else purple, it does not spread anything. A vein in the middle of your house is inert, so you can build around it or use it as decoration without quarantining it. It gives off a faint glow and periodic sparkles, and the Metal Detector flags it ahead of gold, so a stray vein is hard to walk past once you know what you are looking at.