Looks like iron slag or possibly a meteorite to me.
What fyrffytr1 said.
"Slag is the by-product left over after a desired metal has been separated from its raw ore. Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals. While slags are generally used to remove waste in metal smelting, they can also serve other purposes, such as assisting in the temperature control of the smelting, and minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal. In some smelting processes, such as ilmenite smelting to produce titanium dioxide, the slag is the valuable product instead of the metal."
Dave
It is actually rock of some kind... I took a file to the corner and it was difficult but it started to break down after a thin wall of crust to a fine metal looking powder or kinda like a metal saw dust. If that makes any sense.