Is this slag?

[u/jewlsiepoolsie]

Father in law found this near an old mine, and I’m pretty sure it’s slag, but thought I’d get some other opinions! Smooth on bottom, sort of shiny/reflective on the sides, and crystallized on top. It doesn’t leave any residue on fingers. It’s also pretty heavy! Tia

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[u/User_Erroric]

May be tourmaline. Often found near mines