These three little cubes are the first prototypes of yttrium passing the mirror polishing test - and it is almost certainly the first instance anyone has ever taken the trouble to polish this metal. Although not a lanthanide, yttrium is usually regarded as a rare earth metal because its properties fall right in line with the rest of the REEs and is also found mixed with those. It is, from the point of view of metallurgy, pretty much useless. It produces prodigious amounts of sparks when friction is applied turning it into a flammable hazard when machining. It is too rare and expensive to be used in its pure elemental form anyway.

The primary challenge in polishing yttrium to a very reflective luster is therefore to control speed of the polishing wheel in such a way that the cube never becomes heated. It is also susceptible, like most other rare earth metals, to rapid oxidation when moist so needs special dry grit abrasives. This high degree of pain-in-the-assness is responsible for the high cost.