Lanthanum is the black sheep of the rare earth family. It is both the most abundant component of the rare earth ores as well as relatively useless. Nearly all of the demand for it being taken up by toy sparklers and lighter flints. That's because when you grind it the little hot bits that fly off self-ignite in air in a fiery death before they reach the ground. A market for the metal was found in early versions of electric cars that had Ni-Cad batteries, where lanthanum played a role as minor constituent, but Elon Musk et al have snubbed that old tech for lighter lithium.

Lanthanum is also very susceptible to oxidation. Cut a piece in two to expose virgin metal and its luster will dull over the course of a few days until completely black. That thin coating of oxides is, unfortunately, insufficient to stop further degradation so the cubes must be kept stored under oil to stop the reaction from causing further damage. Except even in oil it will slowly continue to deteriorate unless the lanthanum and oil sit inside an airtight container. And by airtight we mean glass or metal as even plastic allows a limited passage of oxygen through.

As is the case with several other reactive metals on this page, we’ve handled the problem effectively by putting the cube inside an ampule with argon which is pretty much the only way you can hold on to this metal in a manner guaranteed to spare its delicate metallic luster while still being able to visually appreciate it.

10mm cube weight is 5.9±0.1g