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At first glance, this hard granular white stone looks like a limestone or marble with many small cavities lined with colourless crystals. Both the rock and the crystals are alunite (potassium aluminium sulphate hydrate). Alunite forms by reaction of sulphuric acid on potassium and aluminium-rich rocks. At Tolfa, the rocks are trachyte lavas, and the acid most probably resulted from the weathering of the iron sulphide minerals pyrite and marcasite. Alunite has been extracted from Monti della Tolfa from the 15th century to the 1940s, to make alum, an important industrial chemical. Alum has been used as a mordant for fixing dye in textiles, for purifying water, in medicines and as a food additive, and many other purposes. However the use of alumstone as a decorative stone is rarely mentioned.
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