Hellstone

Every block you break dumps lava into the hole you just made, and that is the whole difficulty of the Underworld. Mine from the side and stay above wherever the lava will run. Touching the ore burns you on contact, so bring an Obsidian Skull or drink an Obsidian Skin Potion before you start rather than after the first hit. If you have to leave exposed ore beside a walkway, drop Sand on top of it and the contact damage stops.

Spelunker does not light it up and the Metal Detector will not report it, which surprises anyone leaning on both. Dangersense does show it, because the game files it as a hazard rather than an ore. It also cannot be Block Swapped, so replacing a piece means breaking it first and dealing with what comes out.

You need a Nightmare or Deathbringer Pickaxe, which means killing your world's evil boss first, and explosives will not open it up until Hardmode. Once you are down there, smelt before you sell: Hellstone Bars fetch several times what the raw ore and Obsidian bring in separately. The other use for a vein is the lava itself, since placing a block and remining it over and over will fill a container for a trap or a farm.