Cordierite

When transparent enough, cordierite is sometimes used as water sapphires gemstone. Magnesium-cordierite, also called cordierite, iolite or blue silicate mineral,forms as crystals or grains in igneous rocks.It typically occurs in thermally altered clay-rich sediments surrounding an igneous intrusion. It has been found in Tatun, Lanhsu and Lutao, the Lutao is the famous one.