What element has the symbol 'Er'?

The story behind the answer

Er is the chemical symbol for Erbium (atomic number 68), a silvery lanthanide rare-earth element. By contrast, Argon is Ar, Rhodium is Rh, and Einsteinium is Es.

Erbium is famous for its role in telecommunications: erbium-doped fiber amplifiers boost 1.55 micrometer signals in long-distance fiber-optic cables, the sweet spot where glass has minimal loss. It also tints glass and ceramics a soft pink. Memory tip: ER boosts fibER signals — think Erbium equals fiber amplifiers. Its name comes from Ytterby, Sweden, which also gave us yttrium, terbium, and ytterbium.