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Roger Weller, geology instructor                             

wellerr@cochise.edu

Tigerseye
Allison Fisher
Physical Geology
Spring 2006

TIGERíS EYE

 

 

 

 



 

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http://www.bernardine.com/gemstones/tigers-eye.htm

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††††††††††† Tigerís eye is a beautiful gemstone that derives its name from the bands running through it

resembling an eye of tiger.Tigerís eye is found in several countries.It is transformed into jewelry

and believed by many to be a gem possessing power and luck.†††††††††
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††††††††††† Rough Tigerís Eye from Africa††††††††††††† Beadsfrom Africa

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††††††††††††††††††††††††††††††††††††††††††††††† Pictures courtesy of Roger Weller

††††††††††† Tiger's eye (tigers eye, tiger eye, tiger iron) is a member of the quartz group that contains
oriented fibres of crocidolite that have been replaced by silica. It is one of the chatoyant gemstones.
Tigerís eye typically has lustrous alternating yellow or brown bands. It is a classic example of
pseudomorphous replacement. The gems are usually cut en cabochon in order to best display their
chatoyancy.

††††††††††† Chatoyancy is optical reflectance seen in certain gemstones. Derived from the French,
meaning "cat's eye," chatoyancy is from the fibrous structure of a material, like that found in tigerís
eye quartz.The effect of chatoyancy is much like the sheen off a spool of silk; the mobile, wavering
reflection always being perpendicular to the direction of the fibres.For a gemstone to show this
effect it must be en cabochon, with the fibers or fibrous structures parallel to the base of the
finished stone.

††††††††††† Often times because of their names the mineral Chrysoberyl or catís eye is often confused
with tigerís eye.However, Crysoberyl is a beryllium aluminum oxide, which produces the
gemstones alexandrite and cat's - eye.It's tabular crystals (orthorhombic system) are transparent
to translucent, yellowish green to green or brown, and have a vitreous luster. Common chrysoberyl
occurs as crystals or loose, rounded grains in granitic pegmatites, and in metamorphic rocks such
as schist and gneiss. Catís eye is an expensive form of chrysoberyl.†† When polished as cabochons,
these stones exhibit a narrow band of concentrated light along the width of the gem. This effect,
known as cat's eye effect, is caused by inclusions of fine, slender parallel fibers in the gem. The cat's
eye variety may be called "cat's eye", cat's eye, "precious cat's eye", "oriental cat's eye", "catseye",
or "chrysoberyl cat's eye". Many other gems also exhibit a cat's eye effect, but only chrysoberyl's
cat's eye enjoys the privilege of having the name "cat's eye". (All other gems that exhibit a cat's eye
effect must have the gem name preceding, such as "quartz cat's eye", whereas chrysoberyl's cat's
eye is known simply as "cat's eye".)


Tigerís Eye Cabochons

 

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Pictures courtesy of Roger Weller


Catís Eye Cabochon

 

 

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http://www.egemstones.com/catseye.html

 

 

 

 

 

Characteristics of Tigerís Eye

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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http://www.bernardine.com/gemstones/tigers-eye.htm

 

Color:†† Brownish yellow to reddish

Species:†† Quartz

Chemical composition:†† SiO2

Crystal system:†† Hexagonal

Hardness:†† (Mohs scale) 7

Specific gravity:†† 2,66Ī0,20

Refractive index:†† 1,544 -1,553

Birefringence:†† 0,009

Optical character:†† Uniaxial +

 

Tigerís Eye Occurrence

††††††††††† Tiger's eye is mined in Western Australia, South Africa, USA, Canada, India, Namibia, and
Burma and the United States.†† Although tigerís eye is found in many countries around the world,
it is believed that the most beautiful tigerís is found in Africa.

Carved tigerís eye from Africa
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Picture courtesy of Roger Weller

 

History of Tigerís Eye

††††††††††† When rich sources of that precious stone were found in western South Africa in the early
1800s, mineralogists recognized that tiger's eye was a fibrous variety of quartz, or silicon dioxide.
In 1873, the German mineralogist Ferdinand Wibel learned while studying the chemistry of
hawk's-eye, a blue form of tiger's-eye, he found that the gem was almost entirely quartz but that it
also contained fibers of crocidolite, an often bluish, iron-bearing form of asbestos.

††††††††††† At that time it was believed that tiger's-eye was a pseudomorphóa mineral in which crystals
of one material take on the form of another, which it replaces atom by atom.However Peter J.
Heaney, a mineralogist at Pennsylvania State University recently discovered, after studying tigerís
eye, that the crystals that form in tigerís eye do not occur in a pseudomorphic fassion.†† Heaney
found fault-free, column-shaped quartz crystals measuring more than 100 micrometers across and
up to 10 millimeters in length.
What scientists suggests is that the crystal structure of tiger's-eye
forms via a so-called crack-seal mechanism.In this kind ofprocess, quartz and crocidolite crystals
simultaneously condense from hot, mineral-rich fluids coursing through a tiny crack in a rock and
grow to fill it.Repeated episodes of fracturing lead to more cycles of simultaneous, crack-filling
growth of the two crystals.In addition, it was found that crocidolite fibers often ran parallel to the
quartz columns. In some cases, however, the angle between the crocidolite and quartz was as much
as 30 , in those instances, the reflected cat's-eye bands of light were perpendicular to the crocidolite
fibers.Scientists concluded that in tiger's-eye
the chatoyancy arises from the crocidolite fibers, not
the quartz.

††††††††††† After over 100 years scientists feel as if they have finally discovered how tigerís eye is truly
formed.