Promethium

Tool tip
The best thing about promethium is that it emits a luminous radiation. The worst thing about it is that it's radioactive. Handle with caution, unless you would also like to emit a luminous radiation; in that case, imbibe generously.

Promethium is required in vast quantities to craft end-game (as of Oct 2015) structures and gear. It is used to create fercium, which in turn is required for rathumium.

Gathering
Promethium has the longest mining time of all minable minerals. If using a laser pick that is capable of multi-tile mining, such as the HyperDrill or the Special Edition Standard Issue Laser Pick, it is easier to fill in the area around the promethium with dirt and mine the dirt to affect the neighboring promethium more effectively.

A HyperDrill or better is recommended for mining promethium.

Appearance
Promethium is to be considered the rarest of all minerals in the game. It only appears in the desert biome in scarce quantities. Veins of promethium have a teal appearance; darker when compared to cardice. Surface veins are spotty, mixed with rock in a jagged appearance. Deeper veins are more concentrated, taking on a shape similar to other mineral veins.

In-Game before mining : In-Game after mining :