Now meet neodymium of strong magnet fame. Much like any other metal pure neodymium is gray as gray gets. That's pure neodymium. When it comes into contact with air - BAM! - oxygen pairs up with the cube's outermost atoms to form a black oxide. You can get rid of the oxidation with a stiff brush perhaps but, inevitably, it starts to darken and darken and darken some more until pitch black…. and then it starts to disintegrate into an inelegant pile of powder. Sad emoticon :-( 

But despair ye not! We’ve built this glass cage around the fragile neodymium cubes to ensure oxygen can do it no harm. You’ll be able to appreciate the metal’s metallic luster for as long as you don’t drop the thing onto the floor.