Cordierite, DICROITE, or IOLITE,

Cordierite, DICROITE, or IOLITE, a natural silicate of magnesia, alumina, and ferric oxide. It crystallises in stout orthorhombic prisms, and is of various shades of blue, sometimes with a tinge of gray or brown. It exhibits the property of dichroism very clearly, often appearing deep blue when viewed in the direction of the vertical axis, but red or yellow when seen by transmitted light at right angles to that direction.