Painted Pebbles of Southern Tuscany (Italy): A Journey from Natural Beauty to Mineralogical and Geochemical Insights

This study investigates the textural and mineralogical features of unusually coloured pebbles, here referred to as giottoli, collected from hills, riverbanks, and coastal areas in southern Tuscany (Central Italy). Inspired by the curiosity of a non-specialist collector, Roberto Mari, these pebbles were analysed using various analytical techniques to understand the processes underlying their distinctive, art-like patterns. The giottoli display periodic colour banding across a wide chromatic range, mainly in shades of red, yellow, and grey, with varying intensities. X-ray diffraction, elemental mapping, and backscattered electron imaging revealed pronounced mineralogical zonation and significant compositional heterogeneity. Mineralogically, the pebbles primarily comprise quartz, calcite, and phyllosilicates (predominantly kaolinite). Red and yellow zones are enriched in iron oxyhydroxides (FeOx), notably hematite (restricted to red areas) and goethite. In contrast, grey areas exhibit low FeOx concentrations and limited alteration, potentially preserving original mineralogical signatures. The giottoli are interpreted as fine-grained sedimentary clastic rocks undergoing pressure solution processes (stylolitisation) and later mineralisation via carbonate veins. These features may have promoted FeOx precipitation through interaction with iron-rich fluids, likely linked to discharges from the Colline Metallifere mining district (Tuscany). During FeOx precipitation, cations (e.g. Si, Al, Ca, Mn) were incorporated and their abundance (in particular that one of Mn) may be responsible for the outstanding colour variability that makes these natural objects so fascinating. Based on their texture and composition, the giottoli are classified as siliciclastic mudstone. This study illustrates how public curiosity can inspire meaningful scientific investigation, offering new insights into geologic processes.