Mysterious stains in deep end of pool?

[bluskyguy]

I was playing a fetch game with my kids yesterday with swim goggles and when I went to the bottom in
the deep end I noticed several tiny ( BB-size) blackish spots which did not come off when rubbed. In fact
some of them appeared to be eating away at my liner as they felt indented. I managed to pick up a BB size
blackish ball and bring it to the surface. It was very hard. My concern is my liner. Has anybody ever heard or
seen anything like this? I have not added any type of solid chemical for weeks ( salt being the last,but I always
predissolve) and as far as I know the kids havent put anything in. I do have alot of trees around but cant
imagine anything that could stain let alone eat away at vinyl. Thanks

 

[chem geek]

See if it dissolves in a mild acid solution. If it does, and especially if it fizzes when acid is added to it, then it is probably calcium carbonate. Usually that's more gray than black, however, but it could have picked up dirt or something else in the water to become darker. Check your saturation index to see if it's high using The Pool Calculator.