87Rb has been used extensively in dating rocks; 87Rb decays to stable strontium-87 by emission of a beta particle (an electron ejected from the nucleus).
During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase.
82Rb is used in some cardiac positron emission tomography scans to assess myocardial perfusion.
Even though rubidium-85 is more abundant, rubidium-87 has a positive scattering length, which means it is mutually repulsive, at low temperatures.
There is also a strong supply of cheap uncoated diode lasers typically used in CD writers, which can operate at the correct wavelength.