The inorganic chemistry textbooks treat the rare earth metals as if they were all clones of each other. Chemically they're all very alike and this is the main reason why they're so hard to isolate from the ores in which they're all lumped together. The literature, however, could do a better job of pointing out that this doesn't make them interchangeable. Their physical characteristics, for one, are quite differentiated. 

Thulium is one of the rarer of this class of metals and while nothing about it looks-wise is remarkable it turns out that it is especially difficult to prepare into shaped forms. Our supplier, whose specialty is rare earth metals, had no idea that this was the case. No customer had ever asked for something similar before. They were mystified that this metal resisted machining despite being ductile and reasonably resistant to oxidation. Specifically, the various techniques used with the other rare earth metals were not working out in this case. It took weeks of trial and error before the engineers found a solution that could be applied at an industrial scale. And even then, if you note carefully, the surface of the cubes shows speckling and brush marks... vestiges of ill-behaved metal that does not wish to conform into a perfectly cubic shape ;-)

Each 10mm thulium cube weighs 9.3±0.1g.