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.