Didymium

[Sarah]

A rare metallic substance usually associated with the metalcerium; -- hence its name. It was formerly supposed to be an element,but has since been found to consist of two simpler elementarysubstances, neodymium and praseodymium. See Neodymium, andPraseodymium.

[Mark Zuckerberg]

A mixture containing the rare earth elements praseodymium and neodymium, used to colour glass for optical filters. It was originally regarded as a single element.