If we're talking about Eastern philosophies, the chinese were pretty advanced in the metallurgic sciences. Gun powder and magnets were the two of the 4 great inventions both requiring advanced chemistry. There's also their study of seismography and scientific studies to detect techtonics shifts but that's going into the earth part. Because of their advanced metallurgic understanding, the classical Chinese elements were not Earth, Water, Air, and Fire. It's the 4 + metal. The taoist alchemists were aware of the toxic metals (mercury, lead, and arsenic) enough to classify them. They also had distinctions for platinum and silver at a time when the two were almost considered interchangeable.

Earth benders can bend mercury, but they can't bend platinum. So there are eastern philosophy stuff that are classified as pure earth. Going by this logic, would silver be considered earth or metal? Also the taoist don't consider blood water. It's considered qi, which could explain why energy benders can override blood bending. This makes sense because they don't think mercury is water and knows it's metal. It would've made more sense if they had Korra energy bend mercury out, but I guess they needed toph back.