didymium

 
Pronunciation: /dɪˈdɪmɪəm/

noun

[mass noun] Chemistry
  • 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.

Origin:

mid 19th century: from Greek didumos 'twin' (because it was closely associated with lanthanum) + -ium (used as a suffix for new metals)