| Gemstone |
Crystal
System |
Density |
Hardness |
Refractive
index |
Treatments |
| Iolite |
orthorhombic |
2.61 |
7.0-7.5 |
1.542-1.551 |
none |
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Color:
Blue, with gray and slight yellow
modifiers due to its trichroic
nature
Durability:
Care must be taken. Best for beads
and pendants. There is a cleavage
plane along which iolite breaks
quite easily
Localities:
Brazil, Burma, India, Sri Lanka, Mozambique, and Zimbabwe,
The Vikings probably mined iolite from deposits in Norway
and Greenland.
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Iolite
Mg2Al3O
Magnesium Aluminum Silicate
Iolite
(also known as the mineral cordierite), is a magnesium iron
alumino-silicate. The name Iolite was derived from the Greek
word for violet, "ios". It has also been called
water sapphire because it looks very much like a
blue sapphire, and was originally and arguably claimed to
be used for nautical purposes. The story is told that when
Leif Eriksson and the other legendary Viking explorers ventured
far out into the Atlantic Ocean, away from any coastline
that could help them determine position, they used thin
pieces of iolite as the world's first polarizing filter.
Looking through an iolite lens, they could determine the
exact position of the sun, which allowed them to navigate
safely.
These assertions have never been empiracally proven to be
accurate or even possible. There are no archaelogical finds
of navigational tools which incorporate iolite. Therefore,
I view these tales of Vikings and their iolite with skeptism.
The
stone is extremely pleochroic, which means it appears different
colors depending upon which direction the stone is viewed.
Iolite has three pleochroic directions. In one direction,
it appears blue, in another bluish gray and in the other
grayish yellow. It is being used extensively in jewelry
today because it resembles tanzanite
but is much less expensive because it is more plentiful.
Iolite was once thought to have the ability to increase
one's faith. It is the talisman for people named Irene.
Iolite is different colors in different directions in the
crystal. A cube cut from iolite will look a violetish blue
almost like sapphire from one side, clear as water from
the other, and a honey yellow from the top.
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The
following books are my recommendations for further study.
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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
O
18 . 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 SiO
4
tetrahedra. The channels may contain H
2O
molecules. In this configuration, with the exception of the shared oxygen,
a perfect alternation of the Al and SiO
4
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
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iolite, bloodshot iolite,
praseolite |
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blue, greyish blue, violet
blue, grey, yellow, brown, green (rare) |
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some opalescense resembling
star sapphire |
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transparent and translucent |
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crystals, granules, massive |
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distinct in one direction;
imperfect in other directions |
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vitreous and often greasy |
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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. |
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biaxial positive, but
often negative |
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usually strongly trichroic;
Mg-rich shows as pale yellow, pale blue, and violet blue; Fe-rich shows
as colorless and violet. |
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weak bands: 4260, 4360,
4560, 4920, 5350, 5850, 5930, 6451 |
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reacts slightly to concentrated
acids, but more readily to HF. |
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clouds of very small crystals;
zircon crystals surrounded by interference colors rimmed with intense yellow;
hematite platelets in parallel orientation. |
