MINERAL: Iolite (Cordierite)
COLOR: Violetish blue (pleochroic colorless-yellow)
REFRACTIVE INDEX: 1.542 – 1.551
BIREFRINGENCE: +0.045, -0.011
SPECIFIC GRAVITY: 2.61
MOHS HARDNESS: 7.0 – 7.5
India, Tanzania, Kenya, Madagascar, Sri Lanka, Brazil.
Birthstones & Anniversaries
Iolite is the gemstone for the twenty-first wedding anniversary.
According to legend, Vikings used iolite slices to reduce glare when checking the sun’s position.
In legends, ancient Viking navigators used thin slices of iolite as filters to help locate the sun on cloudy days. Whether or not the tales are true, iolite (mineralogists call it cordierite) can be fashioned into beautiful gems. Strongly pleochroic iolite has been incorrectly called “water sapphire,” as it can display a blue to violet hue in one direction and pale yellow to colorless in another.
When tanzanite was first discovered, gemologists initially suspected it might be cordierite, a transparent, pleochroic, violet-blue gem known for thousands of years. Today, cordierite (named after geologist Pierre Cordier) is better known by its trade name, iolite, which comes from the Greek word “ios,” meaning “violet.”
Iolite’s strong pleochroism makes the gem tricky to cut for best color. This in turn continues to pose challenges to producers and buyers interested in promoting the attractive yet problematic gem to retailers as an affordable blue-gem alternative.
This silicate of aluminum, iron, and magnesium has two distinctive features—a beautiful, violetish blue through slightly violetish blue hue derived from iron and a striking, eye-visible pleochroism. Its pleochroic colors differ with its bodycolor. Iolites that appear violet display light violet, dark violet, and yellow-brown pleochroic colors. Bluish iolites display colorless to yellow, blue-gray, and dark violet pleochroic colors. From some angles, then, a bluish iolite can actually appear completely colorless or yellow, and a violetish iolite can look brown.
Iolites are usually cut as faceted gems, but they are also frequently cut into cabochons.
The size range for a fashioned iolite is anywhere from 1 to 10 carats. Fine iolites over 5 carats are rare.
Iolite falls at 7 to 7.5 on the Mohs hardness scale, but given that it has distinct cleavage in one direction, its toughness is only fair. This makes iolite vulnerable to breakage when set in a ring or other setting exposed to rough daily wear.
Unlike tanzanite, iolite is rarely treated. Fine iolite comes by its beautiful blues and violets naturally. 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 one type or another.
Of course, not all iolite shows fine color. Some gems can look grayish or nearly colorless. If the trade were able to treat these less desirable iolites to produce a better, more salable color, it would. However, iolite’s chemistry won’t allow it. Interestingly, the deep blues of some iolites are thought to have the same cause (an iron-titanium charge transfer) as the blue in sapphire. Unlike sapphire, however, iolite can’t be heat-treated to intensify its blue color because its low melting point won’t tolerate the high temperatures to which corundum is routinely subjected.
Because iolite is fairly hard it’s often found in alluvial deposits. In addition to the gem gravels of Sri Lanka, iolite occurs in several areas of Africa, including Kenya and central Tanzania. Other iolite source countries include India, Brazil, and Norway. A significant iolite deposit was discovered in Madagascar in 1994.
Iolite has yet to catch on with retailers as dramatically as tanzanite did in the 1970s and 1980s. Why not? Experts believe that jewelry designers and retailers have yet to see a consistent enough supply of iolite in uniform fine quality to feel confident in ordering a broad and deep selection of the material for their workshops and stores.
According to legend, iolite is called the Viking Compass Stone. It’s said that thin slices of iolite served as glare-reducers and polarizing filters that helped ancient Viking navigators locate the sun on cloudy days. This allowed the Nordic mariners to pinpoint their own location on the seas.
The name iolite comes from the Greek word ios, meaning “violet.” Some believe that the gem aids sleep and helps to unlock creativity. Iolite is often cited as the twenty-first wedding anniversary gemstone.
IOLITE QUALITY FACTORS
Affordable and attractive, iolite can be used in many types of jewelry. It’s sometimes found in large sizes and beautiful, intense colors that satisfy even the most discriminating colored gemstone connoisseur.
Iolite possesses two distinctive features—a beautiful, violetish blue through slightly violetish blue hue derived from iron and a striking, eye- visible pleochroism. Its pleochroic colors differ with its bodycolor. Iolites that appear violet display light violet, dark violet, and yellow-brown pleochroic colors. Bluish iolites display colorless to yellow, blue-gray, and dark violet pleochroic colors. From some angles, then, a bluish iolite can actually appear completely colorless or yellow, and a violetish iolite can look brown. Some gems can look grayish or nearly colorless.
Because it is commonly transparent with relatively few inclusions, iolite is most often faceted. However, iolites can be cut into cabochons or carvings as well. They are frequently seen as beads, especially when the material is of somewhat lower quality.
Iolite is commonly transparent, with relatively few inclusions. It can be found in all shapes and has even been carved to beautiful effect.
Iolite can contain inclusions that cause various phenomena. Occasionally, iolites with long, parallel, tubular inclusions are found. When the cut is oriented properly, these stones yield cat’s-eye gems.
Iolite containing numerous metallic, plate-like inclusions can be cut to display a sparkly effect known as aventurescence. When the platelets are brownish or reddish in color, the gem might be marketed as “bloodshot” iolite.
Size and Weight
The size range for a fashioned iolite is anywhere from 1 to 10 carats, but fine iolites over five carats are rare.