It's Elemental

Rubidium was discovered by the German chemists Robert Bunsen and Gustav Kirchhoff in 1861 while analyzing samples of the mineral lepidolite (KLi₂Al(Al, Si)₃O₁₀(F, OH)₂) with a device called a spectroscope. The sample produced a set of deep red spectral lines they had never seen before. Bunsen was eventually able to isolate samples of rubidium metal. Today, most rubidium is obtained as a byproduct of refining lithium.

Rubidium is used in vacuum tubes as a getter, a material that combines with and removes trace gases from vacuum tubes. It is also used in the manufacture of photocells and in special glasses. Since it is easily ionized, it might be used as a propellant in ion engines on spacecraft. Recent discoveries of large deposits of rubidium suggest that its usefulness will increase as its properties become better understood.

Rubidium forms a large number of compounds, although none of them has any significant commercial application. Some of the common rubidium compounds are: rubidium chloride (RbCl), rubidium monoxide (Rb₂O) and rubidium copper sulfate Rb₂SO₄·CuSO₄·6H₂0). A compound of rubidium, silver and iodine, RbAg₄I₅, has interesting electrical characteristics and might be useful in thin film batteries.