Praseodymium's name preserves a long chapter of confusion. In 1841 Mosander had drawn from cerium ores a substance he named didymium, from the Greek for twin, believing it a single new element and a near-twin of lanthanum. For over forty years didymium was listed among the elements, until Auer von Welsbach demonstrated that it was in fact two elements intimately mixed. He named them for their appearance and their history: praseodymium, the green twin, for its leek-green salts, and neodymium, the new twin. The episode is emblematic of the rare earths, whose stubborn similarity repeatedly disguised two or more elements as one. Praseodymium has no ancient tradition; its poetry is the botanical green of its compounds and the twinship written into its name.
Today praseodymium sits, with neodymium, on most governments' critical-minerals lists, since China controls a large majority of world rare-earth mining and, more decisively, of the specialised separation and refining capacity the two green-and-pink twins still require, a modern industrial dependency that echoes, in miniature, the nineteenth-century laboratories that first had to tease the pair apart. Efforts to build alternative supply chains in North America and Australia remain, as of the mid-2020s, small relative to demand.