PLEAS ADVICE :chrysoberyl or Alexandrite

[Orissa]

I have faceted chrysoberyl, 1pc 24 cts + ,
Tested -BANGKOK ,
EMIL GEMS LABORATORY REPORTS : CHRYSOBERYL
STONES ; CHRYSOBERYL
COLOR ; GREENISH YELLOW
WEIGHT ; 24 CTS +
REFRACTIVE INDEX : 1.746 - 1.755
BIREFRINGENCE : 0.009
SPECIFIC GRAVITY : 3.73
PLEOCHROSIM : DICHROIC - AX , STRONG -X

COMMENTS:: SLIGHT COLOR-CHANGE CAN OBSERVABLE UNDER DIFFERENT LIGHT.

IF COLOR THEN STONE WILL ALEXANDRITE , BUT BKK REPORTS IS CHRYSOBERYL, BUT COLOR CHANGE


ALSO I READ : GEMS : BY WEBSTER READ, FIFTH EDITION:
CHAPTER : CHRYSOBERYL; PAGE NO. 137 : THE HUE OF THESE SRI LANKAN STONES COMPARES FAVORABLY WITH THE ALEXANDRITE, BEING YELLOWISH-GREEN IN COMPARISON WITH THE BLUE GREEN OF THE RUSSIAN. IN ARTIFICIAL LIGHT THE SRI LANKAN STONES ARE BROWNER RED AS AGAINST THEVIOLET-RED OF THE STONES FROM THE URALS. DESPITE THE LARGE SIZES THE SRI LANKAN STONES ARE LESS PRIZED THEN THE STONES FROM RUSSIA
HERE NOT QUESTION OF PRICE , HERE QUESTION THE STONE WILL BE CHRYSOBERYL OR ALEXANDRITE

CAN HELP OR ADVICE


PLEAS ADVICE :chrysoberyl or Alexandrite

MRSGEMS

 

[valeria101]

As far as I know, it is the degree of color change that would ultimately make color-change chrysoberyl warrant the name "Alexandrite". Also, the two colors are expected to be green ad red with some modifier and I suspect that the refference varies a bit among labs. The same naming rule applies regardless of the gem's origin.

However, lareg chrysoberyl pieces are regarded as collectable and presented with notes on origin and unusual characteristics, such as THIS piece. Faint color change qualifies as notable characteristic as well as unusual fluorescence. In such a case, I would expect a label of "color change chrysoberyl" with a note on the degree of color change (say, "20% color change" or "10% color change"). By "color change" I understand change from incandescent to daylight (color dependant on the spectral strength of the light source), not pleochroism.

25 cts and eye clean? Obviously an impressive piece, color change or no color change! By reputation, this size should already be scarce. Is it true ?

 

[Richard M.]

This is like the ruby or pink sapphire question – there’s no clear-cut worldwide rule as to when color-changing chrysoberyl qualifies as alexandrite. The GIA deals with the question this way:

“Chrysoberyl that exhibits a distinct change of color from red under candlelight or tungsten-filament incandescent bulbs to green in daylight or fluorescent illumination is called alexandrite. The name is applicable only when a distinct color change is visible. Many stones are offered as alexandrite, even though they do not have a distinct color change.” Deciding at what specific point the change becomes “distinct” is left undefined.

It should be mentioned there are chrysoberyls, mostly from the Ruvuma region of Tanzania, that show distinct color change in various colors but not red to green-blue. It’s generally agreed among gem dealers that they should not be called alexandrites. They are certainly rarities and should be valued for their own sake but should probably be distinguished from real alexandrite.

GIA also ignores that many types of fluorescent lights exaggerate blue at the expense of reds, unlike daylight. Many alexandrite purists check color change only in candlelight (there were no incandescent light bulbs during the reign of Czar Alexander II) and daylight (no fluorescent lights either).

Good luck with your stone. It sounds as if it's a magnificent specimen!

Richard M.

 

[Orissa]

PLease Advice
COLOR CHANGE CHRYSOBERYL OR ALEXANDRITE

 

[Orissa]

very good your advice, yes definitely we can say color change chrysoberyl .
If color change then Alexandrite
It is true, do not scarce, INDIAN ORIGIN
Where I can Test, they give Alexandrite reports ???
I am sending both picture- color change & Natural
Please advice

 

[Orissa]

Natural Color Chrysoberyl

 

[Richard M.]

All I can tell from the image you posted is that your stone has a huge window. Considering its size and rarity that may be okay with a museum or collector.

As for finding a lab that will grade it as alexandrite, that's where the subjectivity that makes gemology art not science, comes into play. Just as you can sometimes get differing diamond grades from the same lab at different times, you may have the same result with your stone depending on who's doing the grading.

Maybe you should address your question to Mogok, who appears to be off on a field trip again. As an instructor at the Asian Institute of Gemological Science in Bangkok, he's the gem lab expert on this forum but he hasn't posted for a while. He might be able to make a recommendation. The only lab in the U.S. I know that certifies colored stones is Cap Beesley's American Gemological Laboratory in New York. I can't begin to guess what his report would say about your stone.

Richard M.

 

[mogok]

Hello,
This is what I can tell you about the difference between cherysoberyl and alexandrite, in AIGS laboratory we use the following tests:
If the stone is clearly red to green, there is no problem.
Now for stone that goes from greenish to reddish, there are several borderline headhakes. In these case we use the following procedure:

First to be qualified as Alexandrite we must found in the stone some chromium in its spectrum as chromium is the element that gives the color change to alexandrite.
Second Alexandrite is supposed to fluoresce red under UV.

So in border line cases for color change chrysoberyls that show some greenish to reddish color change, if they present at least a weak chomium spectrum and a weak red fluorescence so we can qualify it as Alexandrite.

If they dont we qualify the stone as chrysoberyl.

I hope that I have answered to your question. Please not that for chrysoberyl the evidence of the color change depends a lot of the orientation of the stone: Some stone show no color change in one direction and a visible one in an other direction. All the labs dont have the same procedure regarding to this point. Some check the color change while you look at the stone table, some check for the maximun color change...

Its a little complicated matters as the re is a very important difference in value if the answer is alexandrite or chrysoberyl. So the labs can have sometimes a lot of pressure from their good customers!

 

[mogok]

----------------
On 6/21/2004 2:38:38 PM Richard M. wrote:
Maybe you should address your question to Mogok, who appears to be off on a field trip again. As an instructor at the Asian Institute of Gemological Science in Bangkok, he's the gem lab expert on this forum but he hasn't posted for a while.
----------------

Thanks Richard for these nice words.
I'm not currently on a field trip but I'm very busy with the upgrading of the AIGS website. You will all be able to visit our new website I hope very soon: I'm working hard to make it closer to the school and the laboratory reality: You will be able to discover there many interesting photographs from the field trips I've taken my students in and of course of the gemstones we have the chance to study in our gemological laboratory. I hope that this new version will be helpful for everybody who want to understand more about gemology and gems.
But anyway, you were not that wrong as I'm leaving to Burma tomorrow as there will be next week an important gemstone sale in Yangon!
I will be back soon in pricescope...
All the best,

 

[Orissa]

HI ,

thank you very much for your advice/help, all of you

I really get good Knowdelwg , from this side,

It is true without color change no Alexandrite ,

It's also true , This is like the ruby or pink sapphire question , also Emerald with green beryl

It's fine !!! , Can say color change chrysoberyl

Really I get very good opinions : Some stone show no color change in one direction and a visible one in an other direction. All the labs don't have the same procedure regarding to this point. Some check the color change while you look at the stone table, some check for the maximum color change

.Yes it very important difference in value if the answer is alexandrite or chrysoberyl


Can you inform ? , Bangkok based gemologist address & AIGS website address: