Cordierite is one of the names used for the gemstone that we sell under the name of iolite. It is composed of magnesium aluminum silicate and has a hardness of 7 to 7.5 on the Mohs scale. Cordierite is the name most commonly used in mineralogy, while iolite is used in gemology. The name cordierite comes from the French geologist P. Cordier (1777-1861); the name iolite comes from the Greek root for "violet". For more information see our iolite information page.