Samples prepared in the laboratory containing magnetite and Fe₂O₃ were compared with a Roman bloomery slag from Sharpley Pool near Worchester, which consists of wustite dendrites in a fayalite-glass matrix (anorthite). Wustite forras rounded globular dendrites, Fe₃O₄ well-formed white cubic crystals, and Fe₂O₃ white needlelike crystals. Fe₂O₃ is anisotropic and polarizes dark yellow. The term wustite is preferred to FeO, which implies an exact compound.