didymium

noun

di·​dym·​i·​um dī-ˈdi-mē-əm 

: a mixture of rare earth elements made up chiefly of neodymium and praseodymium and used especially for coloring glass for optical filters

Word History

Etymology

New Latin, from Greek didymos twin, from dyo two — more at two

First Known Use

1842, in the meaning defined above