Rubidium is an element with an atomic number of 37.
In 2375, the Think Tank assisted Saowin's species in stabilizing their homeworld's geology, in exchange for bernicium. However, the bernicium was needed for reconstruction efforts, so instead Saowin attempted to offer a rubidium geode, the only one ever found on their planet. The Think Tank rejected the offer, as rubidium is rare but not unique, and insisted on the bernicium. (VOY: "Think Tank")