Shiny black nuggets of Onyx Ochre look very nice and are occasionally used in ornaments. But the great amount of nocxium is what miners are really after. Like in all else, good looks are only an added bonus.
Dark Ochre was for years ignored as a worthwhile ore to mine until improved refining techniques managed to extract the huge amount of nocxium inside it, the highest ratio of nocxium found in any ore in the universe. 400 ore units of Dark Ochre are needed to refine it.