Even with the enlarged photo, it's hard to say but it kind of looks like granite to me. If it is, it would be fine in your tank.
ive given it a soak in boiling water and a scrub with a new scrubber and its in the tank im just going to keep a eye on my ph
I would think its Granite, a good wash will do. about the only problem you can have with it is, radiation, i kid you not. in some parts of the south west of England, the radiation given off by the granite, is well above the government's "safe limits". ( not just England either, wherever you find granite, you find radiation. a proportion of all granite is Uranium).
It's not like your fish will turn into mutants, or anything like that, but its something worth knowing.