Rubidium oxide is highly reactive towards water, and therefore it would not be expected to occur naturally.
In reality, the rubidium is typically present as a component of (actually, an impurity in) silicate or aluminosilicate.
A major source of rubidium is lepidolite, KLi2Al(Al,Si)3O10(F,OH)2, wherein Rb sometimes replaces K. Rb2O is a yellow colored solid.
The alkali metal oxides M2O (M = Li, Na, K, Rb) crystallise in the antifluorite structure.
The tarnishing process is relatively colorful as it proceeds via bronze-colored Rb6O and copper-colored Rb9O2.