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Chrysoberyl, cat’s-eye, milk and honey, tutorial

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zeolite

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With its silky luster and sharp “eye”, it is the most beautiful chatoyant gemstone in nature. Chatoyant come from the French “chat” meaning cat, and “oeil”, meaning eye.


When the term cat’s-eye is used with no other qualifying term, it means chrysoberyl cat’s-eye. Other cat’s-eye gemstones can include tourmaline, quartz, apatite, diopside, kornerupine and others. The chatoyant effect in chrysoberyl is due to minute parallel cavities. Quartz cat’s-eye, the commonest, owes its chatoyancy and grayish-green or greenish color to parallel fibers of asbestos. The fibers or cavities align themselves to the crystal structure in an orderly manner.


Cat’s-eye is always cut into a cabochon, since only a tall, rounded shape renders the line of the eye properly visible. Here is an analogy of how the eye is formed: Imagine you are holding a spool of thread, and looking past it, toward a bright point light source (the sun). The hole in the spool runs directly away from you, toward the sun, and the threads curve around the spool, running left and right. There is only one angle where the sun reflects off of the thread, directly toward your eye. That one angle is a straight line across the top of the spool, from the sun to your eye. The cat’s-eye works the same way, with the parallel cavities running left and right, over the domed shape of the stone.


An ideal cat’s-eye has a very narrow sharp eye, running completely from top to bottom, with no skips or breaks in the eye. The eye seems to float over the translucent body of the gem. If you rock the stone from left to right in the sun, the eye moves back and forth. If the stone is completely transparent, there are no inclusions, and thus no eye. If the stone is opaque, there can be a good eye, but the eye seems painted on. There is a rare, delicate balance between a good eye and good translucency.


Fine cat’s-eyes can be quite expensive, well into the price range of colorless diamonds. For many years I found them available in only two qualities: low and very high. I didn’t want the low quality, and the very high quality was beyond what I could afford to pay.


4718catsye.jpg
 

zeolite

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The picture below shows the same gem in two different orientations. The left picture has the inclusions running left and right, in the cat’s-eye orientation. The picture on the right shows the same stone rotated 90 degrees, with the inclusions running top and bottom, in the milk and honey orientation. The honey is the dark brown color and the milk is the light, whiter color.





2chryso.jpg
 

zeolite

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The picture below shows the relationship between the light source and the location of the colors. The photo has been rotated to match the drawing. It is counter-intuitive that the dark brown (honey) faces the light source and the milk faces away from the light source, on the shadow or shade side of the gem. It is more logical that the light milk side would face the bright light, and the dark honey side would face the shadow side.

chrysodiagram.jpg
 

zeolite

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While I have read many explanations of the cause of the cats-eye, I’ve never read an explanation of the milk and honey effect. I was thinking about this one day, and my knowledge of optics provided the answer. Remember, you read it here first:


Keep in mind that the cats-eye is translucent and that bottom is reflective. In physics, the law of reflection states that the angle of incidence equals the angle of reflection. The reflecting surface is the polished flat bottom on the cat’s-eye and the angle of incidence is on the sun side of the gem. The diagram shows that the incident light reflects from the polished bottom, and reflects upward on the far (milk) side. It does not reflect back on the near (sun) side of the cats-eye. When the light does reflect upward from the bottom, it is scattered by the many white negative cavity inclusions. This is why the sun side is dark (honey) and the far side is light (milk).


reflection.jpg
 

zeolite

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Another interesting effect is two point light sources, two lamps left and right, with some space between them. In the left picture, in the cat’s-eye orientation, you see two eyes, since each light source traces a different path across the inclusions to your eye. In the middle picture as the stone is rotated either left or right, the two eyes move closer to each other and begin to merge. The right picture, in the milk and honey orientation, shows the two eyes merged into one eye!

3catseyes2.jpg
 

zeolite

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Shown below are three green tourmaline cat’s-eyes. Cat’s-eyes gems are not common at any level of quality, so it is not fair to criticize them. But these three can illustrate some quality issues.


The left green tourmaline has a sharp eye, but it is lacking inclusions at the bottom. It doesn’t have a complete eye. The left and middle green tourmalines have a sharp eye, but no transparency. The eye can’t float across as on a translucent gem. The middle stone has the eye showing left of center. The stone was not oriented perfectly before cutting. The right tourmaline has good translucency, and is the finest of the three, but the eye is not as sharply defined, and the body clarity is not perfect.


The chrysoberyl cat’s-eye on the right shows a sharp defined eye that floats above the wonderful body transparency. Chrysoberyl cat’s-eyes tower in quality over any other cat’s-eye gem!


4catsyes.jpg
 

icekid

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Nov 17, 2004
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zeolite- I always learn so much from you. Thank you for sharing your knowledge with us
 

zeolite

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Aug 13, 2008
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619
Makingthegrade, Icekid,

Thank you for your comments. I not sure how much interest there is here for cat''s-eye, but I think it is quite a phenomenal gem
 

Pandora II

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Aug 3, 2006
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9,613
Thanks Zeolite!

I was very keen to buy one when I was out in SL and looked at as many as possible (which was in the hundreds). Unfortunately there weren''t that many with the body colour I wanted and the perfect eye - and those that were had scary price-tags!

I think the chatoyant gemstones are something that very much appeal to men. My husband was very taken with star-sapphires and has asked me to find enough matched ones to make a set of formal shirt studs and a matching pair of cufflinks! (I''m also working on a set in moonstones). I''ve also noticed a lot of Indian men on the underground in London wearing bezelled catseyes in rings, but never a woman.

Unfortunately it is proving very hard to really find enough information on chatoyant stones to make an informed purchase at the right prices.

Spotting the highest quality isn''t so hard, but the rest I was completely lost! Any tips for resources?

(I did managed to snap up a star garnet for pennies - only 4 armed and with an annoying inclusion, but I couldn''t let it go!)
 
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