The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C60, observed via STM at 300 K, dramatically changes the nanocrystalline shape and fluctuations of the islands. We tune coverage so that a single chain of C60 fully decorates each Ag island boundary, forming a closed circular “necklace.” We model the C60-induced rounding in terms of competing energetic and entropic effects. We thereby characterize the decorated-step fluctuations and estimate the C60-Ag and C60?C60 attractions to be ?0.13 and ?0.03??eV, respectively. Generalizations of our model show that decorating molecules of both circular and rectangular surface-projected symmetry will similarly lower the energy of fully kinked boundaries, leading to corner rounding and reorientations by 30° on (111) surfaces and 45° on (100) surfaces.