Cordierite is a silicate mineral that is found in metamorphic and igneous rocks. It is typically blue to violet in colour and is one of the most strongly pleochroic minerals. Cordierite has a chemical composition of (Mg,Fe)2Al4Si5O18 and forms a solid solution series with sekaninaite, which has a chemical composition of (Fe,Mg)2Al4Si5O18. "Cordierite" is a name used by geologists. When the mineral is transparent and of gem quality, it is known as "iolite" in the gem and jewellery trade. Two older names for the mineral are "dichroite" and "water sapphire." The name dichroite means "two-colour rock," inspired by cordierite's pleochroic property. The name water sapphire is also related to pleochroism. It was used because a specimen could have the colour of a sapphire when viewed from one direction, but if the stone was rotated it could appear to be as clear as water - ""