The causes of deterioration of the marble decoration in the chapel of Saint Eufemia, today extensively detached, were investigated. The decoration was produced in a way similar to a Florentine mosaic (or <i>commesso</i>) by fixing the marble slabs with an adhesive to a slate supporting slab then fixing this multilayer structure to the masonry with iron straps. Mineralogical, petrographic, and chemical analyses by various methods, as well as determination of physical characteristics, were carried out on the various exterior marbles, the supporting slate, and the organic-inorganic adhesive. The results obtained showed a strict relationship between materials, manufacturing technique, and poor resistance to environmental effects.