Dysprosium: another exotic metal few have heard about. It is magnetic, neutral gray and reacts slowly with water tarnishing it into an oxide while freeing water's hydrogen atom. Despite what Wikipedia says about this element, it is most definitely not so soft that it "can be cut with a knife." Regarding rarity it comes towards the rarer end of the scale among the rare earth metals group with which it is associated and, not surprisingly, it is fairly expensive in pure form.

A cube made out pure dysprosium metal has probably never, not once figured on any kid's wish list for Santa but now that Luciteria is making them who knows?

10mm cube weight is 8.6±0.05g