is this gneiss?
The choncoidal fracture and tranlucence indicate glass. What sort of ore refining would result in slag that's made of glass?
Lots of small ore smelting operations produce slag, as do some fires. Many rocks contain silica in some form and fires or smelting often produce a glass component. Typically it has a banded vitreous layer bounded by a cinder-like rind or a welded layer of breccia or grains like the specimen showh above. It is commonly found in this part of the country, and I used to have a collection of slag specimens that I encountered on field trips and other outdoor activities.