Iolite

General
Gem quality iolite approaches the blue colour of sapphire, and used to be inaccurately called water sapphire. It is a complex magnesium aluminium silicate, with the magnesium partially replaced by ferrous iron and manganese, and the aluminium replaced partially by ferric iron. This makes constants for iolite qutie variable.

Other Names
Iolite is also known as dichroite and cordierite.

Colour
The colour normally associated with iolite is blue, and its name comes from the Greek word meaning violet. Iolite also has other colours, which we will explain under the next section pleochroism.

Pleochroism
The differential selective absorption of the ordinary ray and the extraordinary ray in uniaxial stones (dichroism), or of the three rays corresponding to the three principal vibration directions in biaxial stones (trichroism). Iolite is very strongly pleochroichic. The three colours for the three axes are pale blue, dark violet blue, and yellow. This means that it is important for iolite to be cut in the correct direction relative to its axes.

Sources
Ceylon, Burma, Madagascar, India, Canada and California.

Technical Information

Name	Complex Magnesium Aluminium Silicate
Chemical Composition	Mg2Al4Si5O18
Chemical Composition (Alternate)	(Mg,Fe)4Al8(OH)2(Si2O7)5
Hardness	7.0 to 7.5
Refractive Index	1.53 - 1.54 to 1.54 - 1.55
Bi-refringence	0.001
Optic Sign	Negative
Optical Character	Biaxial
Specific Gravity	2.58 to 2.66
Crystalline System	Rhombic