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Iolite

General information

When tanzanite was discovered, gemologists initially suspected it might be cordierite, a transparent, pleochroic, violet-blue gem.

Cordierite (named after geologist Pierre Cordier) is better known by its trade name, iolite, which comes from “ion,” the Greek word for violet.

It’s a silicate of aluminum, iron, and magnesium. Iron causes its violetish blue color.

Unlike tanzanite, iolite has a long history.

  • Iolite’s strong pleochroism makes it an effective light filter.
  • Early Viking navigators used thin slices of it as glare-reducing filters to protect their eyes when they checked the sun’s position.
  • But its pleochroism also makes it a challenge to cut the gem for the best color.

Iolite has two distinctive features—a beautiful violetish blue through violet hue derived from iron, and striking, eye-visible pleochroism. Stones that appear violet body color tend to display light violet, dark violet, and yellow brown pleochroic colors, while bluish iolites display colorless to yellow, blue gray, and dark violet pleochroic colors.

Depending on cutting orientation, iolite can show dramatically different bodycolors.

Cutters try to fashion iolite so that grayish, pale yellow or brownish yellow pleochroic colors don’t show face-up.

From some angles, bluish iolite can actually appear completely colorless or yellow, and a violetish iolite can look brown.

  • These are less attractive colors than blue or violet.
  • And in poorly oriented stones, any face-up brownish or yellow color can lower the value considerably.

Although iolite is sometimes cut into cabochons, it’s most often faceted due to its transparency.

The size range for a fashioned iolite is anywhere from 1 ct. to 10 cts., but fine gems over 5 cts. are rare.

Iolite rates 7 to 7.5 on the Mohs hardness scale, but its toughness is only fair because it has distinct cleavage in one direction.

This makes iolite vulnerable when mounted in a ring or other setting that’s exposed to rough daily wear.

Iolite treatments

Unlike tanzanite, iolite is not treated. Its freedom from enhancement other than normal cutting and polishing is a selling point when customers consider that most blue gems, from inexpensive blue topaz to fine sapphire, receive routine treatment of some kind. Iolite’s freedom from enhancement other than normal cutting and polishing is a selling point when customers consider that most blue gems, from inexpensive blue topaz to fine sapphire, receive routine treatment of some kind.

Iolite Sources

Iolite occurs in Sri Lanka, India and in several areas of Africa, including Kenya, central Tanzania, and the island of Madagascar, where a significant deposit was discovered in 1994.

Other iolite sources include India, Brazil, Norway, and Finland. It’s usually found in alluvial deposits.

After iolite gained more attention in the industry, its prices began rising too, since more retailers were placing orders for the previously unknown gem.

  • Prices for good-quality iolite jumped about 25 to 40 percent.
  • Despite the price increases, however, fine iolite in 2012 sold for about 10 percent of the price of comparable tanzanite.
  • In late 2012, iolite wholesale prices ranged from $5 to $25 per carat for stones weighing 1 ct. to 5 cts.
  • Extra-fine stones in the same sizes ranged from $45 to $80 per carat.