The authors used x-ray diffraction to investigate strain and composition in SiGe nanorings formed during partial Si capping of self-assembled SiGe/Si(001) islands. The obtained results are corroborated with selective wet chemical etching experiments. Clear evidence is provided that rings are composed of a Ge rich core surrounded by Si richer ridges indicating that a substantial material redistribution occurs during the shape transformation from SiGe islands to rings. The results suggest that SiGe ring formation is driven by strain relief.