Didymium, Di, only existed for 44 years. It was discovered in 1841 by Swedish chemist Carl Mosander, extracted from the ore ceria which was already known to contain cerium and lanthanum. As didymium was very similar to these elements, Mosander named it from the Greek word for twin, and Mendeleev placed it in his original periodic table under lanthanum.
Time passed and techniques for identifying and separating elements became more sophisticated. In 1885 Austrian scientist and inventor Carl Auer von Welsbach separated didymium, producing the salts of two distinct elements. The bluish-lilac salt he named neodidymium (new twin) and that with the green salt he named praeseodidymium (green twin). These were soon contracted to neodymium (element 60) and praeseodymium (element 59), and with lanthanum and cerium they constitute the first four elements of the lanthanoids. They are represented here by mirror-image Greek meander patterns.
But the name didymium lives on. It was found that both elements absorbed the brilliant orange emission of sodium, and didymium glass became the material of choice for glassblowers’ goggles, cutting out the sodium glare from the hot glass and allowing glassblowers to see their creations as they form.