A new type of luminescent glass, called "nanocluster glass," has been developed using atomically precise nanoclusters. These glasses are formed from tetranuclear cubane nanoclusters, specifically [Cu₄I₄(PR₃)₄], which are protected by phosphine ligands. The nanoclusters exhibit a reversible crystal-liquid-glass transition, with the glass state showing enhanced optical properties, including high light transmission, low self-absorption, and high luminescence efficiency. The glass is also highly transparent, with a refractive index of 1.714 at 600 nm, and demonstrates excellent X-ray scintillation performance, with a light yield of ~75,000 photons/MeV and a spatial resolution of 30.8 lp/mm. The glass can be processed at low temperatures, making it suitable for applications in photonic devices and imaging. The nanocluster glass also shows potential for use in fiber optics and microresonators, with low optical loss and high flexibility. The study highlights the potential of nanocluster glasses for advanced photonic applications and opens new avenues for the design of novel glasses with specific structural characteristics.A new type of luminescent glass, called "nanocluster glass," has been developed using atomically precise nanoclusters. These glasses are formed from tetranuclear cubane nanoclusters, specifically [Cu₄I₄(PR₃)₄], which are protected by phosphine ligands. The nanoclusters exhibit a reversible crystal-liquid-glass transition, with the glass state showing enhanced optical properties, including high light transmission, low self-absorption, and high luminescence efficiency. The glass is also highly transparent, with a refractive index of 1.714 at 600 nm, and demonstrates excellent X-ray scintillation performance, with a light yield of ~75,000 photons/MeV and a spatial resolution of 30.8 lp/mm. The glass can be processed at low temperatures, making it suitable for applications in photonic devices and imaging. The nanocluster glass also shows potential for use in fiber optics and microresonators, with low optical loss and high flexibility. The study highlights the potential of nanocluster glasses for advanced photonic applications and opens new avenues for the design of novel glasses with specific structural characteristics.