I like the idea of a refining agent for metals, but I'd rather it worked during MELTING them, and was called, you know, flux.
From Wikipedia:
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Some of the earliest known fluxes were carbonate of soda, potash, charcoal, coke, borax,[1] lime,[2] lead sulfide[3] and certain minerals containing phosphorus.
So, out of all these, phosphorus seems to be the most likely candidate. It would be mineable, and since there are 4 different types of it (white, red, violet, black), the violet one would produce a unique-looking ore.
Phorsphorus could also have a wide variety of different uses, beside its main use as a flux: fertilizing plants (most IRL fertilizers hold ample amounts of phosphorus), and if somehow remade into white phosphorus, can be crafted into extremely dangerous incendiary bombs (white phosphorus is known for it's extreme chemical reactivity).
Oh! And it could be used to create stuff that glows in the dark! (Phosphorescence)