Iolite

Gemmological name for gem quality Cordierite.

Iolite shop

iolith rund - Iolite round

Round Iolite in outstanding quality

Origin of name: after the French mineralogist and geologist Pierre Louis Antoine Cordier (1777-1861). Cordier was a member of the Academy of Sciences, one of the founders of the French Geological Society and director of the Museum of Natural History in Paris.

Synonyms and trade names: the mineralogical name is Cordierite. The trade name for gem quality is Iolite. Older trade names, which are rarely used today, are Dichroite, Water Sapphire or Lux Sapphire.

Can be confused with: due to the similar colour, Iolite might be confused with Tanzanite, but in fact Iolite can readily be identified by its unique dichroism, see "Worth knowing" further down.

Localities: Cordierite is a rock forming mineral with occurrences on all continents including Antarctica. The most important sources for gem quality Cordierite, that is to say Iolite, are South India and Sri Lanka. Iolite deposits of lesser commercial importance are in China, Burma, Tanzania and Brazil amongst others.

iolith smaragdschliff - iolite emeraldcut  iolith oval - iolite oval
Two Iolites from Orissa, India

Handling: Iolite is sensitive to heat. Careful during soldering. Apart from that and despite its only imperfect cleavage and good hardness, Iolite is quite brittle and sensitive to pressure. Be gentle during setting. If used as a ringstone, Iolite should be worn with care.

Worth knowing: in one direction Iolite shows a typical bluish violet colour, ranging from watery (hence the old trade name Water Sapphire) to quite dark. If viewed at 90° to the direction of best colour, Iolite appears light grey to distinctly grey, often with a yellowish or light brownish tinge. No other mineral displays this effect as strongly as Iolite and it is therefore diagnostic.

Sometimes Iolites with numerous hematite inclusions can be found. Because of the red colour of these hematite platelets, these stones are called "Bloodshot Iolite" in the trade.

bloodshot iolite
Bloodshot Iolite

Even rarer are Iolite cat´s eyes. Since the foundation of our company in 1987 we were able to buy only one single specimen!

iolith katzenauge - iolite cat´s eye
Iolite Cat´s Eye

Iolite shop

Gemmological Properties of Iolite

Formula:
(Mg,Fe)2Al3[Al2Si5O18]
Crystal system:
orthorhombic
Mohs hardness:
7-7.5
Specific gravity:
2.55-2.77, average 2.65
Refractive index:
birefringent 1.527-1.578
Max. Birefringence:
0.011 to 0.018
Dispersion:
0.017
Pleochroism:
very strong, pale yellow/bluish violet
Luminiscence:
inert
Lustre:
vitreous
Cleavage:
none to indistinct
Fracture:
conchoidal
Colour:
light blue to deep bluish violet