The chemical and mineral composition and formation conditions of dense grey-yellow colour crusts on the surface of monuments and bowls of decorative fountains in Peterhof (Russia) were studied using a wide range of methods (XRD, SEM, EMPA, Raman spectroscopy). The crusts consisted of calcium carbonates (monohydrocalcite, aragonite, calcite), magnesium carbonates (lansfordite, nesquehonite), and Sr-, Pb-rich carbonates (aragonite–cerussite–strontianite solid solution; up to 29 wt% PbO and 43 wt% SrO). Carbonates formed on all types of materials (gold, polyester resin, marble, granite) and do not interact with it. The cause of the formation of carbonate crusts on decorative fountains is the high content of carbonate ions in water. The variety of mineral phases is associated with fluctuations in the pH and Mg/Ca ratio. Recommendations are given on conservation measures that inhibit the formation of carbonate layers that threaten the preservation of the cultural heritage of this UNESCO World Heritage Site.