We recently found some very fine pieces of iolite in the market here, not something that happens very often. That's because it's rare to find iolite in larger sizes (over 1 carat) and with good clarity. High quality iolite is a strikingly beautiful stone when cut well so we wanted to know more about this unusual gem.

Many of the lesser known colored gems are lesser known because they are relatively soft. This is true of gem varieties like fluorite, apatite, sphene and diopside. But iolite is actually quite a hard stone, with a rating of 7 to 7.5 on the Mohs scale, making it as hard as tourmaline and harder than tanzanite, peridot and all the quartz varieties. Iolite is not well known simply because it is so difficult to find. If iolite were commonly available in larger sizes it might be as popular (and expensive) as tanzanite.

Fine iolite is typically a soft violet blue color with unusual pleochroic properties, meaning that it displays different colors when viewed from different angles. Iolite will display a light blue and yellow gray in addition to its dominant hue of violet blue. Gem cutters have to be careful to orient the material correctly so that it will display the violet blue tone when viewed through the top or table of the stone.

Iolite's pleochroic properties were known to mariners who found practical applications for it. The Vikings reportedly used iolite as a polarizing lens, using it to see through the haze and determine the exact location of the sun on overcast days to aid in navigation.

From the gemological perspective iolite is a complex magnesium aluminum silicate with traces of ferric/ferrous iron and manganese. The magnesium is partially replaced by ferrous iron and manganese and the aluminum is partially replaced by ferric iron. Iolite is also known by the mineral name cordierite, after Pierre Louis Cordier, the French mineralogist who first described it in 1809. The name iolite comes from the Greek ios meaning violet.

The value of iolite depends on the violet blue coloration. The richer the blue, the better. Iolite is always untreated; in fact there are no known treatments to enhance the color or clarity of iolite.

Iolite deposits are found in India, Brazil, Canada, Germany, Finland, Madagascar, Mozambique, Burma, Norway, Sri Lanka, Tanzania, the USA, and Zimbabwe. Most of the fine material we have been seeing recently is from mines in Madagascar.