It appears to be either glass slag or metal-refining slag... a byproduct of the refining process, consisting of impurities which are skimmed off of the top of the vat of molten material, and discarded.
Going through a spell of not feeling well again, but speaking up because you said you found your maybe-Flint mystery object in a small stream "30 miles north of Atlanta"... where I was born-&-raised, and did a lot of rock-hounding from there northward into extreme NW Georgia. Natural flint is not found so close to Atlanta... you have to go a fair distance further north for that, up to around Cartersville and beyond.