Lets Talk Gemstones

By Edna B. Anthony, Gemologist

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A Cyclosilicate

Cordierite was known and used as a gemstone in Sri Lanka long before the French geologist-mineralogist Pierre Louis Cordier accurately described it in 1809. Cordierite was identified as a specific mineral and named in 1813. This othorhombic (pseudohexagonal) hydrous silicate of aluminum and magnesium, where ferrous iron replaces some of the magnesium, is the lower temperature form of the dimorphs indialite and cordierite. This higher temperature hexagonal indialite is isostructural with beryl.
The distribution of aluminum is random throughout its rings of (Si,Al)6 O18 . In the ordered six-fold ring structure of cordierite, aluminum occupies the tetrahedral beryl sites, and magnesium and ferrous iron take the octrahedral aluminum sites, while a common oxygen is shared by two SiO4 tetrahedra. The channels may contain H2O molecules. In this configuration, with the exception of the shared oxygen, a perfect alternation of the Al and SiO4 tetrahedra exists in all directions throughout the structure. For this reason, the American mineralogist George V. Gibbs believed cordierite should be classified as a tectosilicate.

This blue gemstone is also known as iolite, dichroite, "water sapphire", "lynx sapphire", and "bloodshot iolite". The Greek word "io", meaning the violet flower, is the source of the name most used. Its distinct pleochroism explains the name dichroite. In Sri Lanka, the lighter colored tumbled pebbles were known as "water sapphires". The term "lynx sapphire" denoted the darker indigo blue stones.

Orientation of the table in faceted stones greatly affects the perceived color of the finished gem. Usually, the optimum color is achieved by placement of this facet perpendicular to the direction where the most intense blue is observed. In volume two of "Precious Stones", Dr. Max Bauer describes a cut that exhibits the distinct pleochroism of iolite. "This object is attained by cutting a cube, the faces of which are perpendicular to the three axes of the crystal. The cube is mounted by one corner on a pivot, so as to show the three different colored faces, and forms an interesting and remarkable object." Some iolites cut en cabochon display opalescense resembling star sapphires. An excellent picture of the hematite platelet inclusions in "bloodshot iolite" appears on page 164 in the "Photoatlas of Inclusions in Gemstones" by E.J. Gubelin and J.I. Koivula.

The ferrous oxide content of cordierite influences its color and causes considerable variation in the specific gravity. With the density, hardness, and double refraction indices so similar to those of quartz, cordierite can easily be confused with quartz. However, these same properties help to distinguish it from other blue gemstones. Praseolite is the unusual leek-green colored cordierite. It is both ironic and confusing that some amethyst from Brazil and Arizona changed by heat treatment to a very similar leek-green color is called Praseolite. Distinguishing between the two could depend on optically locating the interference figures or a more sophisticated analysis. Praseolite (cordierite) is biaxial. Praseolite (green quartz) is uniaxial.

Stubby prismatic gemmy crystals and pebbles occur in the gem gravels of Sri Lanka, Burma, and Madras in India. The especially fine material now comes from Madagascar. Other sources include: Karasburg in Namibia, Babati in Tanzania, Bavaria, Orijarvi in Finland, and the area around Murzinka in the Ural mountains. Gems up to two carats can be cut from some crystals found in Haddam and Guilford, Connecticut. Gemmy nodules occur in Virgolandia and Paraiba of Brazil. Granules are found in regions of metamorphosed pelitic (clay) rock, some igneous rocks, and pegmatites throughout the world. Kragero in Norway and Sri Lanka are sites of vitreous massive material containing the red hematite platelets.

Despite its popularity in Europe during the eighteenth century, iolite is not so well known by the public today. Its pleochroism is especially suited to a brilliant lenticular cut and to a fairly shallow step-cut. Iolite is somewhat brittle, but with its satisfactory toughness and a hardness exceeding that of quartz, it adds a sapphire-hue choice to the array of the less expensive, but more durable stones suitable for rings as well as other jewelry.

Gemstone Properties
(Mg,Fe)2Al 4 Si 5O18
silicate (cyclosilicate)
Crystal System:
iolite, bloodshot iolite, praseolite
blue, greyish blue, violet blue, grey, yellow, brown, green (rare)
some opalescense resembling star sapphire
transparent and translucent
crystals, granules, massive
distinct in one direction; imperfect in other directions
conchoidal and brittle
Fracture Lustre:
vitreous and often greasy
Specific Gravity
varies from 2.53 to 2.78
7 to 7.5
Refractive Index
alpha is 1.522 to 1.558; beta is 1.524 to 1.574; gamma is 1.527 to 1.578; in material from Sri Lanka, alpha is 1.530, beta is 1.534, and gamma is 1.539.
0.005 to 0.018
Optic Character
biaxial positive, but often negative
usually strongly trichroic; Mg-rich shows as pale yellow, pale blue, and violet blue; Fe-rich shows as colorless and violet.
weak bands: 4260, 4360, 4560, 4920, 5350, 5850, 5930, 6451
Color Filter
not definitive
reacts slightly to concentrated acids, but more readily to HF.
Thermal Traits
avoid thermal shock
none known
clouds of very small crystals; zircon crystals surrounded by interference colors rimmed with intense yellow; hematite platelets in parallel orientation.