Iol

Iol, also known as violet stone, was the name by which iolite (cordierite) was commonly known in the Realms.

Description
This semi-precious stone was predominately blue in hue, but the stone's color changed with viewing angle, from dark blue to straw-yellow. Small specimens could be transparent but larger stones tended to have inclusions which could cause a star-like effect or, in the case of trapped hematite crystals, a flash of golden color much like sunstones. Iols were usually facet-cut to best display the stone's color change. A typical specimen had a base value of 50 gp.

Powers
Iol stones were the best component for use in making ioun stones.

Rumors and legends
Faerûnian legends said this gemstone had a strong affinity for magic, but few knew of its connection to ioun stones.