Didymium

It does this by removing part of the orange region of the color spectrum, acting as an optical band-stop filter.

[citation needed] Cerium, lanthanum, and didymium made up least 95% of the rare earths in the original cerite from Bastnäs, Sweden.

In Dmitri Mendeleev's first attempt at a periodic table, the atomic weights assigned to the lanthanides (including didymium) reflect the original belief that they were divalent.

In 1885, Carl Auer von Welsbach succeeded in separating salts of the last two component elements,[5][6] praseodymium and neodymium.

[4] During World War I, didymium mirrors were reportedly used to transmit Morse Code across battlefields.

This was one of a number of decorative glasses using rare earth colorants, with "Heliolit" and "Alexandrit" being the first two, introduced by Moser in 1929.

[citation needed] After a year of further development, the rare earth glasses were introduced to great acclaim at the Spring 1929 trade show in Leipzig.

A typical composition might have been 46% lanthanum, 34% neodymium, and 11% praseodymium, with the remainder mostly being samarium and gadolinium, for material extracted from South African "rock monazite" from the Steenkampskraal mine.