praseodymium (n.)

rare metallic element, 1885, coined in Modern Latin by discoverer Carl Auer von Welsbach (1858-1929) from Greek prasios "leek-green" (from prason "leek;" traditionally identified with Latin porrum "leek," which suggests a PIE *prso-) + didymos "double" (from PIE root *dwo- "two").

The name didymia was given to an earth in 1840, so called because it was a "twin" to lanthana. When didymia was further analyzed in the 1880s, it was found to have several components, one of which was characterized by green salts and named accordingly, with the elemental suffix -ium.