For the first time, the synthesis, luminescent and structural properties of stable perovskite-type (Cs_1−xRb_x)₄PbBr₆ (R = Cs, Rb) nanocrystals are shown. In the absence of rubidium, Cs₄PbBr₆ and CsPbBr₃ perovskite crystals precipitate in the ZnO–Na₂O–B₂O₃–GeO₂ glass matrix. With ascending rubidium content, the precipitation of (Cs,Rb)₄PbBr₆ nanocrystals is replaced by the Rb₄PbBr₆ nanocrystals nucleation. Nucleated nanocrystals exhibit an intense green luminescence. With an increase of the rubidium content, the luminescence maximum shifts to the blue region, the luminescence quantum yield increases from 28 to 51%, and the average decay time increases from 2 to 8 ns. Several assumptions have been made about the nature of the green luminescence of perovskite-like Cs₄PbBr₆ and (Cs,Rb)₄PbBr₆ crystals in glasses. It is concluded that the most probable cause is the impurity inclusions of CsPbBr₃ and (Cs,Rb)PbBr₃ crystals.