|Chemical composition||Complex Mg-Al (magnesium aluminum) silicate|
|Optic nature||Biaxial -|
|Refractive index||1.53 - 1.55|
|Birefringence||0.008 - 0.012|
|Specific gravity||2.58 - 2.74|
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. Due to it’s strong pleochroism, ancient Vikings used iolite for navigation on cloudy days. The gemstone acted as a polarizing filter and allowed the ancient sailors to determine the position of the sun. The soft blue color makes it a very attractive gemstone. Iolite is the 21st anniversary gemstone.
Iolite is usually blue to violetish-blue. Rarely it is colorless.
Iolite is a type II stone in the GIA clarity grading system is usually included. Typical inclusions are:
- liquid feathers
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.
Iolite is very strongly trichroic and this is the primary test in seperating iolite from other gemstones. The three colors you will find are:
- 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.
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 nα = 1.535-1.539 and nβ = 1.540-1.545
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.