Man Made Sandstone

I was at a country fair on the weekend and came across a fellow carving what he called man made sandstone. It was deep yellow in colour with large colour striations to black. He said it was a by-product of the steel casting process. (perhaps fused casting sand) He said it gradually got harder with age. He carved it with tools he had made. Seems he was cutting it up with a hand saw. He would not give up much more information than that. Dose anyone know what this stuff is called? Dose it weather well? Could it be used for outdoor carvings with out a sealer? Â*Thanks