Luminite

This is the only ore you get by killing something rather than mining it, and the Moon Lord hands over a fixed amount per fight. Committing to one fragment line, meaning a single armor set with its wings, pickaxe and tools, takes only a handful of rematches. Trying to build everything takes closer to twenty. Decide which set you want before you start farming, because the wrong order costs you several fights.

Nothing detects it. Spelunker, the Metal Detector and every other treasure sense ignore it completely, which hardly matters given it never appears in the ground. Any pickaxe mines the placed blocks, so there is no tier gate on building with it. Everything crafted from it glows faintly, and that glow shows through vanity, which is worth knowing if you care how your character looks.

If you finish a playthrough sitting on spare bars, crafting them into Void Monoliths before selling returns substantially more than selling the raw material.