A comprehensive evaluation of the primary bioreceptivity to phototrophic biofilms of several varieties of granite was carried out. The bioreceptivity of the granites was more strongly affected by the physical characteristics of the stones than by their chemical composition. Growth of phototrophic biofilms was strongly enhanced by high open porosity, capillary water content and surface roughness. The findings of the study enabled us to develop a robust and well-founded bioreceptivity index (BI) for granitic rocks. The proposed BI has two components: BIgrowth, which quantifies the extent of the biological growth, and BIcolour, which quantifies the colour change undergone by the stone due to the colonisation and which can be considered the bioreceptivity perceptible by the human eye. The index was fitted to a qualitative scale of 0-10 and can therefore be used as a decision-making tool for selection of appropriate lithotypes for building and/or ornamental purposes
