Rare Earth Elements (Ce, Nd, Dy, Er, etc)

Neither rare nor earths, these 17 elements are nonetheless difficult and unpleasant to mine and refine. Chemically near-identical, these metals have unique magnetic and optical properties, making them essential in modern technology from fibre optics to wind turbines. So should we worry about China's stranglehold over their supply?

(Picture: 20 euro note glows under an ultraviolet light; Credit: Frans Dekkers/Thinkstock)