Malachite

Malachite is a brand new ore added in Alpha 7. It only appears on the surface, and there's one per mine. It glows an aqua color, so it's easy to locate right when the mine resets.

Trivia

 * Malachite is the only ore to not spawn at all. It only gets created when the mine resets, so very few players actually have it.
 * Despite this, it isn't the only ore that spawns when the mine resets. Azurite spawns in the first 5 layers, so it can also spawn when the mine resets.