Iolite

Cordierite forms in thermally metamorphosed clay-rich sedimentary rocks, in high-grade schists, gneisses and granulites, and also in some igneous rocks. Because iolite is fairly hard it is often found in alluvial deposits, such as the gem gravels of Sri Lanka. Most iolite in world commerce comes from India; other sources include Tanzania, Kenya, Brazil, Norway and Madagascar, where a significant deposit was discovered in 1994.

The most useful sign is eye-visible pleochroism: a bluish iolite can look blue-gray, dark violet, or colorless to yellow depending on the viewing direction, and a violet one can look brownish from some angles. Its refractive index (about 1.54–1.55) and specific gravity (about 2.61) fit cordierite. Iolite is rarely if ever treated, because heating cannot improve its color, so its color can normally be taken as natural. The name "water sapphire" is incorrect.

With a Mohs hardness of 7 to 7.5 iolite resists scratching well, but its distinct cleavage in one direction gives it only fair toughness, so it can break from a hard knock, especially in rings. Do not clean it in an ultrasonic cleaner; GIA lists iolite among gems sensitive to heat and temperature changes. Warm water, mild soap and a soft brush are the safest cleaning method.

Cordierite was named after Pierre Louis Antoine Cordier (1777–1861), the French mining engineer and geologist who first studied it; the trade name iolite comes from the Greek "ios", violet. According to legend, iolite is the "Viking compass stone": thin slices are said to have served Viking navigators as polarizing filters to locate the sun on cloudy days. GIA notes that whether this is true is unknown. Iolite is often cited as the gemstone for the 21st wedding anniversary.

Some people believe that iolite aids sleep and helps unlock creativity, as GIA reports; these are beliefs, not tested properties.