Without any background information or knowing the environmental conditions on where you found this piece, all we can do is guess?
My first thought was iron ore slag.
"Slag is the glass-like 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.
Basic slag is a co-product of steel-making and is typically produced either through the blast furnace - oxygen converter route or the electric arc furnace - ladle furnace route. To flux the silica produced during steel-making, limestone and/or dolomite are added, as well as other types of slag conditioners such as calcium aluminate or fluorspar. The major components of these slags therefore include the oxides of calcium, magnesium, silicon, iron, and aluminum, with lesser amounts of manganese, phosphorus, and others depending on the specifics of the raw materials used."