Cobalt's blue compounds were used in ancient times to colour glass and ceramics. Jewellery of five thousand years ago has been found to contain such pigments. The element itself, however, was discovered by the Swedish chemist, Georg Brandt, in 1735, and was recognized as elemental by T.O.Bergman in 1780.
Rhodium was discovered in 1803 by the English chemist, William Wollaston, who dissolved crude platinum in aqua regia, and extracted the element from the resultant residue. Iridium was isolated in the same year, from the same source, by the English chemist, Smithson Tennant. Both elements were named in recognition of their coloured compounds: rhodium, the salts of which tend to give a rose-colour in solution, derives its name from the Greek word {Irhodon}, meaning 'rose'; and iridium, the compounds of which are variously coloured, derives its name from the Latin word {Iiris}, meaning 'rainbow'.
