Metal clusters consisting of only a few atoms have different physical properties than single atoms and larger nanoparticles. Like molecules, they have discrete energy levels that allow electronic transitions; these transitions are the source of their distinct properties (1). To maintain their special features, clusters must be protected against aggregation and chemical modifications. Scientists have developed water-dispersible clusters that are stabilized with the help of short peptides, proteins, DNA, or polymers (2). Clusters may also be stabilized in solid-state matrices such as glasses, zeolites, or ultracold noble gases. On page 686 of this issue, Grandjean et al. (3) elucidate the origin of silver-cluster luminescence in a sodalite zeolite, a simple model that can withstand high-energy irradiation and displays bright green luminescence. They link the properties of the emitted light to different cluster structures in the zeolite.