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I am trying to take this stuff off the rock because it looks gross and unsightly to say the least. I don't even know what this stuff is but it has eaten into the rock and formed these sores that I have no idea how to remove. I tried bleach.

The stuff sometimes glows under UV light and these rocks have about 30% moisture according to my pinless moisture meter. Is this mold or some kind of algae?

Slate with red discolouration

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    That looks as if it is iron deposits in the stone. I think only grinding will take them out Doing so may well expose more deposits that are deeper. More grinding may compromise the integrity of the stone.
    – RMDman
    Commented Dec 21, 2022 at 18:46
  • you can not selectively remove those. You can bleach, or use diluted muriatic acid, but both will make the whole surface duller.
    – DIY75
    Commented Dec 21, 2022 at 19:21
  • If bleach did nothing, it's not organic, and thus not mold. I would spray saturate some CLR or lime-away on it and let it soak and drip. If the drips are red colored, then you have rust. or cadmium, but likely just rust which you can dissolve out with the spray.
    – dandavis
    Commented Dec 21, 2022 at 19:22

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Flip the stone?

I agree with RMDman. Those red deposits are in the stone. They have been there for longer than humans have been walking. But maybe you could remove the stone, flip it and put it back? Same stone but I bet those red deposits are not through and through. The back side might look nicer.

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  • Likely to be very difficult to remove because iron will be starting from black oxides in the rock and weather /oxidize in air and water to red "rust". Rust is rather complicated mix of iron oxide hydrates and hydroxides. So newly exposed black oxides will turn red-brown.
    – blacksmith37
    Commented Dec 21, 2022 at 20:57
  • @blacksmith37 it does not look like adjacent rocks have those spots but they are probably from the same quarry. Do you think the rock in question will have oxide spots on the back too?
    – Willk
    Commented Dec 21, 2022 at 21:14
  • Probably, it takes time for the red/brown oxides to develop.
    – blacksmith37
    Commented Dec 22, 2022 at 19:04
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Yeah but if you seal the stone after the flip you may slow oxidation. F9 FARC is the best rust remover you will find. It's a commercial product so do exactly what it says. Much safer that muriatic and other acids. You could use a film forming sealer on the iron then a matte sealer over the whole surface after flipping. Might as well use the bad surface to test everything and then flip.

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