16.41: Iolite

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Page ID: 4725

gemology
Gemology Project

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Iolite
Chemical composition	Complex Mg-Al (magnesium aluminum) silicate
Crystal system	Orthorhombic
Habit	Massive, prisms
Cleavage	Distinct, basal
Fracture	Sub-conchoidal
Hardness	7.5
Optic nature	Biaxial -
Refractive index	1.53 - 1.55
Birefringence	0.008 - 0.012
Dispersion	Low, 0.017
Specific gravity	2.58 - 2.74
Lustre	Vitreous
Pleochroism	Strongly trichroic

Figure \(\PageIndex{1}\): Marquise Shaped Faceted Iolite, courtesy of Richard Kocerek.

Iolite is also named "water sapphire", cordierite or dichroite. Iolite has been worn for centuries and derives its name from “Ios” - the Greek word for violet. It is often claimed, that due to its strong pleochroism, ancient Vikings used iolite for navigation on cloudy days. It is conjectured that the gemstone acted as a polarizing filter and allowed the ancient sailors to determine the position of the sun. Although this is a widely published story, there is no empirical archaeological evidence that actually supports it.
The soft blue color makes it a very attractive gemstone. Iolite is the 21st anniversary gemstone.

Iolite is a color variety of the mineral cordierite. Sometimes the name "cordierite" is used to indicate "iolite".
The green variety of cordierite is named praseolite.

Diagnostics

Iolite may be confused (due to its color) with sapphire and tanzanite.

Color

Iolite is usually blue to violetish-blue. Rarely it is colorless.

Magnification

Iolite is a type II stone in the GIA clarity grading system is usually included. Typical inclusions are:

liquid feathers
hexagonal platelets of biotite, hematite or pyrrothite

File:Iolite inclusions1.jpg

Figure \(\PageIndex{2}\): Hexagonal platelets, liquid feathers
60X Magnification
By Barbra Voltaire

The iolite pictured above features liquid feathers, internal cleavage planes, and randomly oriented hexagonal platelets, possibly biotite or pyrrothite.

Optical and physical properties

Although iolite is usually confused with sapphire and tanzanite due to its color, its optical properties will distinguish iolite from them easily.

Pleochroism

Iolite is very strongly trichroic and this is the primary test in separating iolite from other gemstones. The three colors you will find are through dichroscopic examination are:

violet-blue
pale-blue
pale yellow

Sapphire is dichroic and although unheated tanzanite is also trichroic like iolite, the heated tanzanite is dichroic. Natural unheated tanzanite is very rare.

Refractometer

Iolite is biaxial with a negative optic sign. The β ray is almost halfway between α and γ so precise readings should be taken.
A typical reading will be n_α = 1.535-1.539 and n_γ = 1.540-1.543
The maximum birefringence ranges from 0.008 to 0.012
Tanzanite is also biaxial but has a positive optic sign and its values are too high to indicate iolite. Sapphire is uniaxial and the readings are also too high.

Quartz has RI readings in the range of iolite, but is uniaxial.