Crystal system Orthorhombic
Transparency Transparent to translucent
Luster Vitreous
Fracture Uneven to sub-conchoidal
Cleavage Distinct pinacoidal
Specific Gravity 2.61
Hardness 7 to 7.5
Optical Character Biaxial - ; Double Refractive
Refractive index 1.542-1.551
Birefringence 0.008
Pleochroism Strong;
Violet stones: light violet, dark violet & yellow-brown.
Blue stones: colorless to yellow, blue gray & dark violet.
Chemical Formula Mg2Al4Si5O18

Iolite (EYE-oh-lite) is another mineral that is important in nature but rarely sufficiently transparent to be of interest as a gem material. The most attractive colors, and the only ones used for gem purposes, are blue and violet. It may also be colorless, yellowish white, green or brown. The name of the species is from the Greek meaning "violet". It has also been called by a number of other names, one of which is cordierite (CORD-ee-er-ite), after Pierre Louis Antoine Cordier. Another is dichroite (DYE-kro-ite), from the Greek word meaning "two colored", because of its strong pleochroism. Unfortunately, the misleading name "water sapphire" has been applied to the mineral. Cordierite is the term used most frequently by mineralogists.

Iolite is a hydrous silicate of aluminum and magnesium, corresponding to the formula Mg2Al4Si5O18, and it occurs in short, prismatic, orthorhombic crystals. Its hardness is 7 to 7 1/2, its toughness ii only fair (due to distinct cleavage), and its fracture is uneven to sub conchoidal. The value for specific gravity is 2.56 to 2.66, and the R.I. is usually 1.542 - 1.551; however, it has been reported as low as 1.534 - 1.540 and as high as 1.592 - 1.597. In the laboratories, no reading above 1.555 nor below 1.540 has been encountered. Birefringence is approximately .008. There are no characteristic inclusions. Iolite is biaxial negative, with the intermediate index .001 to .003 from the high figure, and its luster is vitreous. Although the dispersion is feeble, the pleochroism is strong; light violet, dark violet and yellow-brown for the violet variety, and colorless to yellow, blue-gray and dark violet for the blue type. A phenomenal effect in the form of a weak star is infrequently present; chatoyancy also is rarely seen. Iolite is fusible and is attacked by acids. The low R.I. and S.G. and strong, characteristic pleochroism serve to separate it rather readily from the stones it resembles in appearance.

Sources for this mineral include Ceylon, Brazil, India, Burma, Madagascar, Finland and Norway. It is usually found in gneiss and crystalline schists and in zones of contact metacorphism. It also occurs as a primary constituent of igneous rocks.

Translucent material is cut in the cabochon form and transparent material is faceted; in both cases, care must be exercised to orient the stones for best color. No particular precautionary measures are indicated. 42° crown angles and 43° pavilion angles are satisfactory. Iolite responds well to either tin oxide or Linde A on a tin lap.

Because of its attractive color and fair durability, it has possibilities as a jewelry item: however, the supply of clear stones is sporadic and the name is not well known, either to jewelers or to the public. Iolite is more commonly seen in dealer's stocks than many of the other unusual stones. Occasionally, jewelers ordering sapphires will receive iolite in error.