What Is On This Rock

[Malawi MaD]

I have these rocks in my tank but there is a few i want to put in but im not going to till i know what this is, The rock is limestone but what is this brown/rusty colour on the rock. Its not fuzzy or anything looks like colouring but dont no if it could release anything into the tank. Any help would be much appreciated

 

[Malawi MaD]

Could it be copper? Doesn't dis colour the water when its left in a bucket either.

 

[KittyKat]

Most likely iron… copper would not be that colour.

 

[Malawi MaD]

Ok May add them to the tank, they don't effect the water in any way apart from raising the PH which is good for what i need them for.