Cordierite

Cordierite is a silicate mineral that is found in metamorphic and igneous rocks. It is typically blue to violet in color and is one of the most strongly pleochroic minerals. "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 jewelry trade. Two older names for the mineral are "dichroite" and "water sapphire." The name dichroite means "two-color 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 color of a sapphire when viewed from one direction, but if the stone was rotated it could appear to be as clear as water. https://www.mindat.org/min-1128.html https://geology.com/minerals/cordierite.shtml