Cobalt Ore

Every world rolls this or Palladium Ore as its first Hardmode metal, and you do not get a vote. Whichever one you have is what the first tier of Hardmode gear gets built from. If you want the other, crates, a second world, or the Chlorophyte Extractinator are the ways across.

None of it exists until you start smashing altars. The first break seeds this tier, the next two open the tiers above, and the cycle repeats with steadily less ore each time around. That is an argument for restraint rather than for clearing every altar you find. Each break also drops fresh blocks of the evil biome and the Hallow somewhere underground at random, and doing all of them in one session turns a manageable spread problem into a permanent one. Break three, go mine what appeared, come back when you need more.

Nothing below a Molten Pickaxe will touch it, and ordinary explosives bounce off, so the usual trick of blasting a vein open is out unless you have the Power Bomb line. The blue reads badly against wet cavern stone, so a Spelunker Potion is worth more here than it is for the orange metals.

Real cobalt metal is a dull silver. The blue comes from cobalt blue, a pigment made from cobalt oxide, which was one of very few mineral blues available to painters and porcelain makers and expensive enough to be treated as a luxury good.