Answer: Based on your description, it sounds as though some mineral in the stone may be oxidizing or reacting to exposure to the air (perhaps a pollutant of some sort) or to some other element, perhaps through rainfall. You might want to contact a stone specialist or geologist who could help you identify the stone and what is causing the reaction, then you might be able to determine a way to prevent or counteract it. A stone specialty supplier might also be able to offer some insight, if it is a type of stone they carry. I'm sorry I can't be more helpful.
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