Cordierite is a rock-forming mineral with the composition Al3(Mg,Fe)2[Si5AlO18]. It belongs to the orthorhombic crystal system. The crystals have a prismatic form, and crystal twinning is common. The aggregates are blocky and granular. Its colour is light blue, light yellow, grey or brown, and it often has several distinct tones. It has colourless streaks, transparent to translucent diaphaneity, a vitreous lustre, a specific gravity of 2.53–2.78, a Mohs hardness of 7–7.5 and conchoidal fractures. It is a metamorphic mineral that forms in metamorphosed rocks rich in aluminium and magnesium. It is commonly found in hornstone and sometimes in crystalline schist. Cordierite with beautiful colours can be used as carving materials.