"Rock" ID

[arenkay]

This is an example of some sort of rock that I found while vacationing at Lake Cumberland. I think it is some sort of concrete or at least something man made. Maybe to stop erosion of the lake? I was hoping someone else would have some info for me and of course the big question is to whether it would be safe in my tank! I have tried cleaning it with everything imaginable and the color will not fade or rub off. My opinion is that they wouldnt line the edge of the lake with something toxic. would they? of course your dealing with 150 gallons compared to billions...

 

[neekaragua]

i dont think it would be any different then putting a background that has been coated with concreet and cured
those pieces have probably been in the lake for 10 or more years

 

[Mystix212]

Doesnt look like any igneous or metamorphic rock so I'd say its just concrete. If you've found it in a healthy body of water and it looks like it's been in there for at least a couple of months, just clean it with hot water first then dunk it in.