marine safe rock?

[Reef Puffer]

there is some weird mineral build up by a local lake and i find this stuff on occasion just lying on the beach. im assuming now its a sort of tufa rock(ty elias). im sure it is "growing" locally. it looks like really nice rock. very porous and like i said FREE. seems to bubble quite a bit with vinegar. so the way i see it.... it could b safe and at the same time good for my water great on my bank account and eventually a great live rock, or on the other hand one bad trace mineral and can b the death of my entire system.... Hmmmmm... thats what i thought. how many would take the chance?? fortunately my tank is cycling tho and maybe i can just b cautious with the first livestock? maybe just put it back in the garden? Grrrrr idk? o well thanks alot guys, still undecided whether to gamble or not tho.