Reidite is the most powerful resource players can obtain in the game (so far, not including quest items). It is found only in the lava biome, and there are only around 5 deposits on the map. Because of this, reidite is considered the second rarest mineable resource, beaten only by emeralds. It can only be mined with an Amethyst Pickaxe or a Reidite Pickaxe.
For more information, see The Lava Biome.