By a selective staining method combined with UV fluorescence microscopy, gypsum can be detected even in small amounts in calcite sandstone. In thin-sections, gypsum shows a striking orange color. This method gives an insight into the dynamics of gypsum formation, because it reveals to what extent and in which crystal-size the new deposits are formed. The reaction of atmospheric sulfur dioxide with the stone becomes vividly clear and makes it certain that sulfur dioxide, along with various other causes of natural erosion, plays an additional and meaningful role in stone corrosion.