Cordierite crystals, a selection of specimens of this crystallized mineral species. Cordierite is a mineral with many collector’s items presented here with a description, one or more photos and sometimes a video. These cordierites are for sale. It is a silicate, subclass of tectosilicate, with the chemical formula Al3Mg2AlSi5O18 with traces of Mn, Fe, Ti, Ca, Na and K, orthorhombic crystal system.