Didymium

Didym, abbreviation Di, has long been viewed as a chemical element until it was Per Teodor Cleve recognized by the composition of two elements which were in 1885 by Carl Auer von Welsbach finally isolated as praseodymium and neodymium. From Didymerde also samarium was recovered.

1839/1840 was Carl Gustav Mosander in mineral Cerit next to the companion cerium lanthanum and just Didym (Greek twin ( lanthanum ) ).