rubidium - a soft silvery metallic element of the alkali metal group; burns in air and reacts violently in water; occurs in carnallite and lepidolite and pollucitemetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. carnallite - a white or reddish mineral consisting of hydrous chlorides of potassium and magnesium; used as a fertilizer and as a source of potassium and magnesium lepidolite - a mineral of the mica group; an important source of lithium pollucite - a rare mineral that is an important source of cesium |