Black Permanent Deposits on Backyard Rocks

Asked June 18, 2020, 7:06 PM EDT

I have numerous large quartz-like boulders in my backyard which all have varying degrees of black mould/lichen/fungus on them (see attached image). I don’t know how many years it has taken to develop (I am a new owner) but it is impossible to remove. Chemical agents (eg bleach, paint stripper, TSP) have no impact, nor does physical action (eg wire brush etc). Really curious to find out what it is. Interestingly it is not visible on rocks that have been buried under soil. Thanks for your help. Nigel

Montgomery County Maryland

1 Response

Hi Nigel. This is an interesting question. We answer home gardening questions but this is outside of our area of expertise. The coloration looks like it is part of the mineral content of the rock, but we cannot say for sure.