Crystallization of 62.5GeS2–12.5Sb2S3–25CsCl glass ceramic was investigated using different techniques from the macro- to nanoscale. A two-step crystallization process was evidenced, allowing for a direct correlation between the microstructural features and the evolution of the mechanical and optical properties. This two-step crystallization process starts with the nucleation of spherical CsCl crystals that progressively evolve toward a disc-shaped morphology when they grow. This transformation goes along with a local enrichment of Sb, leading also to a significant increase of the lattice parameter of the CsCl phase. These crystals significantly improve the mechanical behavior of the glass with only a small reduction of the infrared transmission properties.