Discovered in 1861 as an impurity associated with samples of the mineral lepidolite, it is used in photocells and vacuum-tube filaments, and made headlines in January 2001 when Harvard physicists slowed light to a stop in a vapor composed of it.
One atom of it, when combined with five atoms of silver and four atoms of iodine, has the highest room temperature conductivity of any known ionic crystal.  Discovered in 1861 as an impurity associated with samples of the mineral lepidolite, it is used in photocells and vacuum-tube filaments, and made headlines in January 2001 when Harvard physicists slowed light to a stop in a vapor composed of it. Discovered along with cesium by Kirchoff and Bunsen in 1859, what is this alkali metal with atomic number 37, which, FTP, is distinguished by its bright red spectroscopic lines?
One atom of it, when combined with five atoms of silver and four atoms of iodine, has the highest room temperature conductivity of any known ionic crystal.  
rubidium