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