Metal Stain

I have stains on my grout where my tile is. The stains also appear to be in my flagstone coping. I have one piece of coping that is cracked from the stain. The worst spot is moving to the plaster.

I have several other stains that are developing. All have dark spots/stain on the coping and a stain on the grout right under the dark spot on the coping.

I rubbed some PH Down on the grout and the stain got lighter.

Most of the stains are above the water line.

It seems to me that what ever is happening that is causing the stain is comming from the flagstone coping.

Has anyone heard of flagstone coping releasing metals that stain the pool?

What else could this be?

here are some pictures from another post.

I cross posted this after I learned this is a metal stain from the acid test.

http://www.poolforum.com/pf2/showthread.php?t=1842