atomic number 55, caesium, cesium, Cs - a soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal

atomic number 37, Rb, 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 pollucite

mineral - solid homogeneous inorganic substances occurring in nature having a definite chemical composition