Caesium: the essentials. Caesium is known as cesium in the USA. The metal is characterised by a spectrum containing two bright lines in the blue (accounting for its name). It is silvery gold, soft, and ductile.It is the most electropositive and most alkaline element.aesium is known as cesium in the USA. The metal is characterised by a spectrum containing two bright lines in the blue (accounting for its name). It is silvery gold, soft, and ductile.