Did you ask the wall builder to identify the discoloration? I have laid a lot of Wisconsin limestone over the years but have not seen anything similar to this materialize in just a few days. Hope you get an answer and post it. Good luck!

I'm pretty sure it is sandstone, not limestone. The landscaper thought it might be moss but I disagree. It seems to be within the stone, like a mineral reacting with the rainwater sitting on top. What should I do? It's getting worse. Even if they clean it somehow, isn't it just going to keep appearing?

This isn't the best photo. It's a chip I took off close up.

You might ask the landscaper to ask his stone supplier. Someone up there who works with this stone has seen this condition before. It is so hard to say from a photo, but again, I have never seen anything like this materialize as a result of rain. It looks like a chemical reaction but it's not as if the stone hasn't been previously exposed to the elements. This has my curiosity piqued and I am going to ask our local stone supplier. Can you email me a pic to jalgozzini@kdlandscapeinc.com I'll see what I can find out for you. John Algozzini

In the short term I googled "Lichens on Sandstone". If you perform that search there are a gazillion photos, a few of which resemble your situation. Certainly not harmful and if they are lichens developing it will add character to your stone wall over time.

Thank you, I will try cleaning with bleach as recommended. It seems though that lichens spread out from a central spot, while the green on my stone is scattered randomly. Also if it were lichens, why aren't all the stones affected? None of my nearby limestone is affected. Just about half of the sandstone retaining wall. There seems to be no rhyme of reason which ones are affected, such as location or direction they are facing.

I tried cleaning that chip with diluted bleach and it literally disintegrated. It broke up into pieces and then became a paste. The bleach didn't do anything to remove the green...I just ended up with green paste. Did I get a bad batch of stone? What am I supposed to do now that it is installed? This wall cost a ton of money. I get that it's natural stone and stuff like moss and lichens add to the character...except that the "character" is only on some of the stones. Others look fine.

The staining looks like mineral oxidation - when exposed to air and water, several minerals will oxidize into other colors. Copper is the most common mineral to produce blue-green coloring. It is quite natural and really nothing you can do about it.

But it is NOT a lichen moss or algae. They don't look anything like that and could not have appeared that rapidly anyway. And they would easily have been washed away.

I agree it is probably a mineral. Did I get a bad batch of stone? Will it ever get better? What should I say to the landscaper? I am really not happy with the way it looks.

I like it myself. Maybe because it's a bit unique.. What's the word?...Patina!
It looks like mineral oxidation. As Gardengal mentioned above, copper.
Mike...sometime rockhound

Hi...I have the same problem with my retaining wall...did you find any answers?