The chemical, mineralogical and main physical properties of 20 samples belonging to the polygenic sedimentary “Agnano breccias” were determined within the framework of a wide-ranging research study on the building stones and mortars used in the cities of Pisa and Lucca (western Tuscany, Italy) throughout the Middle Ages. Data was drawn from unweathered samples collected in abandoned quarries near the village of Agnano (Mt. Pisano, Pisa, Italy). The analysed breccias exhibit a wide range of macroscopic features and chemical and physical properties. Nonetheless, all the analysed samples can be construed as belonging to a single compositional “family”, whose individual members are mixtures in different proportions of a carbonatic component and a silicatic one. Agnano breccias were widely used during the Middle Ages, when they were worked into square blocks and used as an inexpensive, low-grade building material. Minor quantities were quarried up to the 19th century. Their uses and the deterioration processes to which they are subjected are also briefly described.