|I have a North-facing patio whose stones (real stone rather than concrete slabs) have gradually developed many small black spots over the last 10 years, with spot sizes ranging from 1mm to about 10mm. Because of the north aspect, they need cleaning regularly which I do with a jet washer, but this doesn't remove the spots. Nor does disinfectant, nor does thick bleach (slight improvement perhaps but no real removal). Scrubbing ineffective --it's in the surface of the stone. Any idea what the spots are, and any suggestions on how to get rid of them? They are making the stones look really unsightly now.