The composition of chromite is (Mg,Fe) Cr2O4, and it is an important chromium ore mineral. It contains 76.91% Cr2O3, and it has an isometric crystal system. The crystals are octahedral and always appear as blocky, granular or nodular aggregates. The colour is black or brownish black with dark brown streaks. It has an opaque diaphaneity, a metallic lustre, a specific gravity of 4.5–4.8, a Mohs hardness of 5.5 and uneven fractures. It usually forms in magmatic rocks, especially ultramafic and mafic rocks. Sand placer deposits also often contain chromite.