Some rocks in the mines are grey, some are red, some are different shapes? Obviously they are there for the sake of variety, but are some more valuable than others? Are red rocks more likely to drop coal for example? (I am not referencing rocks that have minerals obviously sticking out of them.)
The grey rocks only loot normal rock, atleast they do before the last patch, I didn't play enough to secure that in the actually patch do the same. On my mine time, which it is a lot, the different colours rock can give you: rocks, geodes, coal or "normal" ores like copper, gold or metal. Also, the different colours means what type of geodes can you get. On the first levels on the mines you can get "Geode" on the coloured rock and with the red rock on the finals levels can loot you the red geode. As far I played, all the coloured rocks have a chance to also loot Omni Geode. A coloured rocks never can loot minerals but monster or The Quarry, when you "repair" the bridge on the community center, can do. But on the mines, yes, different colours means different loot.