Wikipedia, Oct. 27, 2012 (Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group; Rubidium has only one stable isotope, ⁸⁵Rb, with the isotope ⁸⁷Rb, which composes almost 28% of naturally occurring rubidium, being slightly radioactive with a half-life of 49 billion years; naturally occurring rubidium is composed of two isotopes: the stable ⁸⁵Rb (72.2%) and the radioactive ⁸⁷Rb (27.8%))

Emsley, J. Nature's building blocks, 2011 (Rubidium; Rb; soft white metal and a member of group I (row 5S) of the periodic table, and they are collectively referred to as the alkali metals; there are two naturally occurring isotopes: rubidium-85 which accounts for 72 percent and is not radioactive; and rubidium-87 which accounts for 28 percent and is radioactive, with a half-life of 49 billion years; it is only weakly radioactive, but it does contribute to the Earth's background radiation)