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mineral data
Mineral Name: cordierite
Gem Names: cordierite - iolite - dichroite - water sapphire
Chemistry: Mg2Al4Si5O18
Class: silicate - cyclosilicate
Crystal Sys: orthorhombic
Hardness: 7 - 7.5 Specific Gravity: 2.3
Fracture: conchoidal Refractive Index: 1.542 - 1.578
Dispersion: 0.017
Misc Prop: Iolite is sometimes called dichroite, but it is, in fact, a trichroic gemstone. This means it show three colors, one for each of he crystal axis. The strong color shift is between the blue and the other two (similar) colors. The blue axis is usually oriented to he top of the stone so it appears dark blue form the top. The other two axis are pale yellow and a gray.

The blue color is very similar to many sapphires and thus it has also been called "water sapphire". Because of he secondary colors in the iolite, the blue is often tinged with gray or yellow and is not as pure as the better sapphires. Unlike sapphire the color cannot be improved with heat.

It has reached a degree of popularity in the gemstone industry but remains a special or collectors item more often than not. The non gemstone material is used as a ceramic in catalytic converters due to its very low thermal expansion.

Specific Images:
cut stone and rough with 3 colors shown