Chatoyancy

Chatoyancy is a term hatched from the French phrase “œil de chat,” which means “to shine like a cat’s eye” (the name stems from the way a cat’s pupil narrows to a thin slit under bright light).

Certain cabochon cut gemstones exhibit this optical phenomenon. Light reflected from such gemstones forms a band of light just beneath the surface. The light band sashays on the stone as you move the gem under a concentrated source of light.

2.34 Carats Chrysoberyl Cat's Eye

2.34 Carats Chrysoberyl Cat’s Eye

Chrysoberyl Cat's Eye 1.60 Carats

Chrysoberyl Cat’s Eye 1.60 Carats

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Two different words are used to describe the effects arising out of the reflection of light from a gemstone. ‘Sheen’ describes the effect caused by the reflection of light from underneath the surface of the gemstone as opposed to ‘luster’ wherein the light is reflected from the surface.

We have coined a number of terms to describe various phenomena associated with sheen and caused by light reflecting from beneath the surface of a gemstone. ‘Chatoyancy’ is one of them. (Asterism, Iridescence, Labradorescence, Adularescence, Aventurescence, and Opalescence are other such terms). They occur because of interference, reflection, or refraction of light as it interacts with the inclusions or internal structures within the gemstone.

Prominent examples of gemstones displaying Chatoyancy are Cat’s-Eye Chrysoberyl and Tiger’s-Eye (Quartz with asbestos fibers). Quality specimens of Cat’s-Eye Chrysoberyl display the finest chatoyancy. Genuine gem enthusiasts show a keen interest in gemstones that display ‘chatoyancy’ or simply, ‘the cat’s eye’ effect. The scrumptiously good-looking ‘milk and honey’ cat’s eye chrysoberyl & the rare cat’s eye Alexandrite are highly coveted.

Chatoyance requires a profusion of thin threadlike parallel inclusions (known as a “silk”) contained within the gemstone. Light scattering from these inclusions forms a thin band across the surface of the gem. The band of light always shines perpendicular to the length of the parallel inclusions. Such inclusions can occur in the form of crystals, hollow tubes, holes, channels, or other linear formations. Inclusions proliferating the stone are usually aligned with a crystallographic axis parallel to the base. Needle-like crystals of hematite & rutile are recognized for producing a cat’s-eye in many specimens.

A greater profusion of fibers (inclusions) produces a more well-defined cat’s eye. Best cat eyes are clearly visible, they symmetrically bisect the cabochon, the eye contrasts sharply with the stone’s body color and moves smoothly as the stone is turned under a source of light.

10MM Genuine Natural Green Chrysoberyl Cat Eye Beads

10MM Genuine Natural Green Chrysoberyl Cat Eye Beads

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Role Of Titanium Oxide Needles (Rutile), Or Silk Inclusions:

Asterism and chatoyancy have an essential aspect in common. Titanium oxide needles (rutile) or silk inclusions in the crystalline structure are the factors that usually trigger asterism. However, the very same crystalline impurities spark chatoyancy if we cut the stone in a chosen direction in the crystal.

Fibrous Growth In The Crystalline Structure:

Furthermore, the growth structure of the mineral itself can also generate the cat’s eye effect. For example, certain minerals will spawn the cat’s eye if fibrous growth occurs in the crystalline structure.

An excellent example of such a stone is the cat’s eye nephrite jade. This gemstone is resplendent with interlocking tremolite fibers in a preferred orientation. Additionally, the gem-cutter should cut the stone into a cabochon. Under such conditions, the interplay of incident light with the fibrous structure induces chatoyancy in the jewel. And we will have our cat’s eye effect running perpendicular to the growth of the fibers.

2.30 Carats Chrysoberyl Cat’s Eye

2.30 Carats Chrysoberyl Cat's Eye

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Hollow tube inclusions:

Hollow tube inclusions perform the task of fibrous growth in other gemstones like beryl. Aquamarine, morganite, and occasionally even emerald (gems from the beryl family) typically host long hollow tube inclusions. The cut stone displays a cat’s eye if the lapidary correctly orients these tubes while cutting en cabochon. The phenomenal band of light runs perpendicular to the direction of the tubular inclusions.

And yet, cat’s eye chrysoberyl is the numero uno so far as this phenomenon is concerned. We can only refer to this particular stone as a cat’s eye without any suffix. So if you hear someone calling a gem ‘ cat’s eye,’ it is cat’s eye chrysoberyl. On the other hand, you have to use suffixes such as cat’s eye aquamarine, cat’s eye quartz, cat’s eye Jade, etc., with other stones.

A blue streak inside the cat’s eye identifies the cat’s eye chrysoberyl (or just the cat’s eye). Of course, you will not always see the swathe of blue, but you will see it very often. And it confirms that the gem is chrysoberyl.

Milk and Honey Effect:

Chrysoberyl can also display the coveted milk and honey effect. Particular cat’s eye stones play host to this yet another gorgeous phenomenon. Here, the side of the stone farthest from the light source turns milky & translucent. Hence, it is the milk side. On the other hand, the side nearer to the light source appears transparent and is the honey side.

Cat’s eye also manifests itself in moonstone and a few species of quartz. Thus, we see that nature blesses many different genera of minerals with the cat’s eye effect. The natural world achieves this by inserting the requisite inclusion clusters into various minerals. When we compare this with asterism, we see that the star effect can be birthed only by needles or long plate-like inclusions.

However, other factors like the intensity of the band of light, the clarity attribute of the gemstone, and a desirable body color taken together determine the value of the stone.

Other cat’s eye exhibiting gemstones are green and pink Tourmaline, many varieties of Quartz, Moonstone, Aquamarine, Apatite, Scapolite, Tanzanite, etc.

Natural Cats Eye Faceted Onion Briolettes, Green Cats Eye Beads, Chrysoberyl, Cats Eye Necklace

Natural Cats Eye Faceted Onion Briolettes, Green Cats Eye Beads, Chrysoberyl, Cats Eye Necklace

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The Eye of the Lion Cat’s Eye Chrysoberyl:

In the late 1800s, a rough crystal of cat’s eye chrysoberyl weighing over seven hundred carats was discovered in the gem-rich gravels of the Pelmadulla rice paddy fields in Sri Lanka. The field was owned by a famous aristocratic lady Iddamalgoda Kumarihamy. Her legally adopted grandson inherited the invaluable gem and decide to cut it. He summoned the most famous gem cutter in the region & personally oversaw the shaping of the stone. The result of these painstaking efforts was an astounding 465-carat cabochon. Authenticated by the Smithsonian Institute this is the largest cut example of Cat’s Eye Chrysoberyl in the world. The family fittingly named the gem ‘The Eye of the Lion’ to honor the symbolic lion that graces the Sri Lankan flag.

Paul E Desautels, curator of the division of Mineralogy at the Smithsonian Institute in Washington articulated his desire to acquire the ‘Eye of the Lion’ in 1978. However, the gem disappeared completely from public view for nearly three decades. It finally re-emerged at the AGTA New York Laboratory in September 2006 with the gemstone dealer Jeffery Bergman of Primagem, Bangkok, Thailand.

Chatoyancy in Colombian Emeralds See Light of Day.

Misael Angel Rodriguez and Claudia Beltran Rubiano are husband and wife duo gem-cutters from Columbia. Together, they came across an opportunity to buy a huge lump of chipped trapiche emerald in the rough in September of 2012. And they almost passed on the chance. The 370 carats rough was cloaked in a carbonaceous coating. Just a hint of green hinted at the probability of an emerald underneath. 

A number of other experts had already bypassed the bid. It meant that buying the lump was an expensive gamble. The sizable piece seemed to have been mined from Muzo, a reputed repository in Colombia. Yet, it was hiding its secret from probing eyes.

Cutting reveals the Emerald Mystery.

On the one hand, host rock inherently covers chunky pieces of rough emerald, and buyers indulge in speculation to find a quality deep color gem inside the containing rock.

On the other hand, what lies within remains an unknown factor till the stone is sliced open. Finally, sellers do not allow the rock to be cut even slightly for fear of damaging or destroying the gem within.

Rodriguez & Claudia Beltran Rubiano put together their experience as gem cutters. Consequently, they decided to take the plunge and purchased the rough.

The duo cut open the stone at corners and saw that the piece contained two portions of a six-ray trapiche emerald. A thorough analysis determined that they could carve two similar-sized gems from it. equally important, a fine tube-like formation appeared at one side of the rough. It was a classic characteristic of material creating cat’s-eye gemstones. Finally, as they exfoliated the skin, they exposed the indisputable green of fine emerald.

Further Analysis Discoveries and Decisions:

Slowly and skillfully, they started removing the carbonaceous outer layer of the emerald. Their knowledge told them that trapiche emeralds often yielded cat’s eyes gems. This was further affirmed by the fine tube-like contours of the gem revealed earlier. As such, they placed small, curved facets on the rough’s surface. They did so to ascertain the orientation of the cat’s eye. They soon understood the optimum positioning of the cat’s eye and planned the cut accordingly. 

The pair of gorgeous long cabochons started taking shape under the expert hands of the husband and wife team. The pair of polished emeralds weighed a total of carats. In order to minimize the inclusions, some parts of the stone had to be polished away reducing the weight of the gem pair to 75 carats. 

The Gemological Institute of America requested to see the stone after the  Tucson gem show of February 2013. The stones were then sent to the Carlsbad laboratory for further scrutiny.

GIA Laboratory Examination

GIA found the gemological properties of these sibling gems to be typically the same as those of cat’s-eye emeralds examined in the past. They recorded a spot RI of 1.57, chrome lines in the red portion of the spectrum, and no reaction to ultraviolet light.

The dense parallel growth tubes (that gave rise to chatoyancy) appeared slightly transparent. This was quite unlike the needles or growth tubes that cause the chatoyancy in many other cat’s-eye specimens.


See Also

Phenomenal Gemstones Asterism in Star Gemstones
Adularescence and Labradorescence Gemstone Fluorescence

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