Crimtane Ore, and its counterpart Demonite Ore, can be mined extremely early-game with Bombs. They are both very rare, so this won't get much, but the small veins can add up over time. And since the ore glows, it can easily be seen even in unlit stone.
If one is looking to make money, selling the Crimtane Ore dropped by the Eye of Cthulhu and Brain of Cthulhu will yield considerable amounts of gold. This method works at any point in the game, and gets better as the player progresses.
One Crimtane Bar sells the same as 3 of the ore (39). No value will be lost or gained when converting ore to bars.