Asterism on a Green Stone?

[katbadness]

Hi,

I have a curious question. Is there such a thing as asterism on a green colored stone (emerald, tourmaline, garnet, etc.)?

My understanding is that asterims is has nothing to do with the color as much as the internal structure of the mineral itself. I have heard of asterisms on corundums (star sapphire, star ruby, even star yellow sapphire). I have also heard that this phenomena is displayed on other types of stones (quarts is the one I have heard). But in all my research, I have yet to hear asterims being displayed on something like.. an emerald for instance.

Further Net browsing also yields no result for other green stones.

The reason behind my question is because I'd love to give my bf a star . He loves green cabochons and he loves asterisms. And I thought something like a star emerald or star tsavorite garnet will be a cool gift for him.

Any thoughts?

 

[DiamondExpert]

Asterism in emerald is apparently not unheard of, but I have never seen one or even a picture of one...chatoyancy (cat's eye) is, of course, not common but better known.


You could consider a green sapphire with asterism, but most greens are pretty ugly in my opinion - I have never seen one of these either.

Another, not altogether serious, path might be quartz with asterism stained green -

- ya, OK I agree, forget it

 

[valeria101]

If you ask about yellow or green star sapphires, most will just assume you must be joking! In spite this, they are out there... only not that many. You would have a good chance to find one from an Australian source: these are sort of a local celebrity which has yet to cross borders allot. One such sources could be simplysapphires.com and their Australian connections. Sure Garry could know more of these... No idea where to find a better piece online (this is a 35cts cab), but they can come in smaller sizes and different shades, transparent stones included.

Emerald cats eye can be great, but stars are hardly "true" stars: no idea whether there are any with more than a secondary, faint eye crossing the first. The stones look like cats eye emerald with some extra glow. If a cat's eye stone would do, such emeralds are a true royal treat though.

Star quartz... maybe, but "green" in this stone means "yellowish olive" and the asterism is never apparent without directional lighting.

 

[pyramid]

Not an asterism but thought I would attach this link.

http://www.yourgemologist.com/emerald.html


I believe these are collectors pieces and not for sale but there is shown the very unusual pair of star trapiche emeralds and also a trapiche emerald.

I have seen on the internet others buying trapiche gemstones but do not know how expensive or rare they are. Maybe one of the experts here could say?

I don't know if you would say a trapiche is a star but it is similar I think.

Pictures of both are on the above link.

 

[mogok]

Hello,
Wow Ana! Wow wow wow...

Thats a fine piece! The best I've seen so far may be. Very nice...
Do you known were it is from?

 

[valeria101]

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On 2/20/2004 6:37:41 PM mogok wrote:



Do you known were it is from?
----------------

Australia, but no idea of the precise location though...

 

[katbadness]

Thank you for the feedback folks! I really appreciate it.

I'll share 2 pictures that started this whole asterism on a green stone discussion between me and my bf.

He saw this star ruby picture:

and also this picture:

.

And then he said... Hmmm... I wonder if there's a star emerald or jade out there.

 

[mike04456]

Aha! That explains it. That second stone is not a star (asterism) but a cat's eye (chatoyancy). They are basically the same thing, but have different names because of the appearance--the only difference is the number of rays. 1 = chatoyancy, 2+ = asterism.

 

[mogok]

Hello,
Yep, Law Gem is right...
Asterism begins when 2 lines cross each other. A green gemstone that ca show 4 rays asterism is green diopside from Mogok. But ususaly green diopside are found as nice cat's eye stones. Star diopside are usualy more brownish blackish.
I've heard one or while about emeralds with star but was never able to see one.
For jade there is not such effect. Jadeite as nephrite is an agglomerate of very small crystals. You dont have there the orientated needle inlcusions or tube that give chatoyancy or asterism.
Star peridot is possible, I've got one specimen... It is quite a rare stone in Mogok but I've seen several. Anyway the star is quite weak. Its more a curiosity for gemologists.
For tourmaline, tsavorite, demantoid, chrysoberyl I dont know about asterism. But chatoyancy is of course possible for chrysoberyl (an even alexandrite) and tourmaline.

All the best,

 

[mike04456]

I too have "heard" about star emeralds but have never seen one or even a photo of one. If one were to actually exist and be reasonably attractive (color-wise), I would expect it to be quite expensive as a collector's stone. I doubt it would ever reach the consumer market.




I have seen star garnets but not demantoids or tsavorites. As mogok says, I don't think star jade is possible because it is polycrystalline.

 

[mogok]

Hello,
I get back the photo of my star peridot... It was on a native Burmese ring and displaying a 4 rays star ( weak but visible). For reference the stone was 84 carats. The star in peridot is usualy not visible in small stones...

All the best,

 

[valeria101]

That's one nice emerald. Here's one user friendly source of them. If you stll wander about star emeralds, there is a good chance that these guys know more.

Star peridot! This is a new one...

Cats eye jade (nephrite) is not... No idea what causes the "eye" - it surely is not due to inclusions, but the estructure of the material itself). The material occurs both in Canada and Russia, and is more rock shop curiosity than gem. I believe that the Russian material is still nephrite, not the jadeite of the Urals or Central Asia.

 

[mike04456]

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On 2/24/2004 1:37:29 AM valeria101 wrote:





Cats eye jade (nephrite) is not... No idea what causes the 'eye' - it surely is not due to inclusions, but the estructure of the material itself). The material occurs both in Canada and Russia, and is more rock shop curiosity than gem. I believe that the Russian material is still nephrite, not the jadeite of the Urals or Central Asia.
----------------

Yet another demonstration that every time you think some gem material only occurs a certain way, someone will come up with a sample that blows up all those assumptions.

 

[katbadness]

Star peridot! WOW! That's a nice one, too.

And cat's eye nephrite!! That is also very different.

Hmmm..... Thank you for the references and help guys. I agree with LawGem..

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On 2/24/2004 12:02:50 PM LawGem wrote:


Yet another demonstration that every time you think some gem material only occurs a certain way, someone will come up with a sample that blows up all those assumptions.

----------------

Well, given the oddity of nature... I will patiently look for one, then.

My bf did say that if one can't be found, he'll settle for a star ruby! HAH! "Settling" for a star ruby... I know his taste, he'll want one that is transparent like shown below, but with a much stronger star.

Anyway, in the meantime, thought I'd share with you generous guys and gals another curiosity of nature that I found.

 

[MichelleCarmen]

----------------


My bf did say that if one can't be found, he'll settle for a star ruby! HAH! 'Settling' for a star ruby... I know his taste, he'll want one that is transparent like shown below, but with a much stronger star.


----------------

Ah, that ruby ring is to die for!

Michelle

 

[mike04456]

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On 2/24/2004 1:03:36 PM katbadness wrote:





My bf did say that if one can't be found, he'll settle for a star ruby! HAH! 'Settling' for a star ruby... I know his taste, he'll want one that is transparent like shown below, but with a much stronger star.

----------------

Well, then how about this one:

http://famousdiamonds.tripod.com/rosserreevesstarruby.jpg

This is the Rosser Reeves star ruby, a 138.72 ct stone. Unfortunately, it's not for sale, being part of the National Gem Collection in the Smithsonian.

 

[katbadness]

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On 2/24/2004 1:37:51 PM LawGem wrote:


Well, then how about this one:

http://famousdiamonds.tripod.com/rosserreevesstarruby.jpg

This is the Rosser Reeves star ruby, a 138.72 ct stone.
----------------

That is one *HUGE* ruby... Wow...
Quick!! Don't let the bf see that one!! Otherwise I'd be eating ramen for the rest of my life!

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On 2/24/2004 1:37:51 PM LawGem wrote:


Unfortunately, it's not for sale, being part of the National Gem Collection in the Smithsonian.

----------------

Thank GOD it's not for sale... Phew! I'm saved from myself again, apparently.

 

[valeria101]

Star ruby: good luck... going down to the source to find any decent one not listed on the stock exchange yet!

That (or a simmilar) yellow star was once posted on cherrypicked.com is this it? A certain shop semed to have all kinds of star sapphire I could think of and then some: on the web it goes under the name of UkGems although their site in not terribly representative of the business...

 

[DiamondExpert]

...and don't forget star spinel...not too common, but not expensive...

 

[mogok]

Hello Gary,
Red 6 rays star spinel?
Are you kidding?
The only star spinels I've heard about are mined in the Thabeikgyin area in the west of the Mogok valley in Burma and they are very dark brownish with a 4 rays star that is not really obvious to see.
The stone you present looks like a synthethic stone...
Did you check it or submit to a laboratory?
If its not that expensive, please just tell me where I can find such thing... I would like to study it in the gemological laboratory I'm working in...

All the best,

 

[Nicrez]

What causes these stars? Is it the chemical composition, like the appearance of a certain element, or is it more atomical, like a physical structure of the atoms? Just curious if it's something that is uniform in all the stared stones, or if each stone has it's own case that causes it...?

 

[mogok]

Hello,
The star in star ruby and sapphire is coming from the reflection of the light on rutile (titanium oxyde) needles. It is a mirror effect. This is the reason why the star looks to move. These needles were exsolved of the ruby or sapphire structure during its cooling process. As rutile ans corundum dont have the same structure, the miscibility is quite low. At high temperature rutile can be in solution but the solobility goes down rapidly as the temperature decrease.
The rutile needle will crystalise as needles in specific areas within the corundum structure. These needles are all the time orientated with 60 degre angles to each other. This is the reason we found 6 ray star corundum.
Other than rutile hematite can also create 6 rays star effect in corundum. But in this case there is 30 degre angle difference with the star create by rutile.
This is the reason we can find very rare 12 rays star sapphire that have 6 rays due to rutile and 6 rays due to hematite. The star in this case see all the rays connecting to each other.
An other possibility of star stone is twin star where 2 crystals grow together with a different axis and direction. If there are rutile needles then they will crystalise in each crystal. We will then see a stone that have 2 6 rays stars with different centers...
The last type of star corundum is the trapiche star. Here the star affect is not create by needles but by other inclusions. In trapiche stars the star is static and does not move.

That the story for corundum. Now in other stones like garnet rutile crystalise in 2 direction to the star will not be 6 branch but 4...
Many stones can show star effect: Ruby, sapphire, garnet, quartz, peridot, diopside,...

Here are to illustrate better inclusion photographs I did in my laboratory in ruby and sapphire:
The first one show the typical internal world of a Mogok unheated ruby with short rutile needles in nest like pattern, along with euhedral crystals (probably apatite)
The second show rutile needles like we found usualy in Sri Lankan sapphires: Long rutile needles. But to be true this photo was taken in a Mogok Unheated Burmese Sapphire.
The third and the fouth photo are detailled rutile needles in a Mogok ruby and sapphire. You can clearly see the typical triangular shape of the needles with re-entrant angles and their disposition at 60 degre to each other.

All the best,

 

[katbadness]

Mogok beat me to it.

Here's a simple description that I found.

"In gemmology, an asterism is an optical phenomenon displayed by some rubies, sapphires, and other gems of an enhanced reflective area in the shape of a "star" on the surface of a cabochon cut from the stone.

Asterism is caused by dense inclusions of tiny, parallel, slender fibers in the mineral which cause the light to reflect a billowy, star-like formation of concentrated light which moves around when the mineral is rotated. This is usually caused by small needles of rutile (titanium oxide) in the case of ruby and sapphire. The stars may exhibit four, six, or more rarely twelve rays. The rays are reflections from the rutile crystal. Rutile can also cause a "cat's eye" effect in gemstones. Some specimens may display much stronger asterism than others, and some specimens may have areas where the inclusions are not present, leaving holes or empty areas in the star."

 

[valeria101]

I didn't know there is virtually no star spinel in Burma ! The ones below are suposed to come from Ceylon and at $30 a piece I did get some under a microscope for fun. I never took pictures of my stones (I have no hardware for it) though. The pictures attached come from the net (Cresla.com)...

 

[mogok]

Hello,
Well that's a fact Ana: I dont know about star spinels in Burma except this kind of brownish unatractive stones from the Thabeikyin area (difficult to find because mining there is not really allowed and if I remember well the area is very controlled by the army that has some mining there...)
For the rest to illustate better the explanation about stars here are some visual examples:

Photos from first to last:
A 12 rays star sapphire: All the 12 branches connect to the same center: 12 branches due to rutile needles, 6 branches due to hematite platelets. This one is from Mogok, Burma.
A twin star ruby: You can see 2 6 rays star with 2 different centers due to the fact that the cabochon was cut in a twinned stone. This star ruby was mined in Luc Yen, Vietnam.
The you have then a trapiche star ruby with a static star from Mong Shu, Shan State, Burma
Finally you have a 2 trapiche sapphire with also here static stars. Both stones are from Mogok, Burma

All the best,

 

[mogok]

Ooops...
Sorry I've forgotten the photo in my precedent post!
Here are...

 

[DiamondExpert]

I just learned that garnet (I think pyrope and almandine, but not sure) also can have asterism!...4 and, rarely, 6 ray stars!

They are found only in Nothern Idaho, www.fs.fed.us/ipnf , and India...got this from the May issue of Rock & Gem.

You can go hunt them yourself for about $10!

...learn something every day...

 

[Richard M.]

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On 3/18/2004 9:55:03 PM DiamondExpert wrote:

I just learned that garnet (I think pyrope and almandine, but not sure) also can have asterism!...4 and, rarely, 6 ray stars!

I've dug and cut many of them, D.E. Learning how to find the star and cut them with the apex centered on top involves quite a learning curve!

Idaho produces the finest star almandines in the world. Four-ray material is pretty common but six-ray stones come from an area where the Forest Service generally won't allow digging. Good six-ray stones are hard to come by.

Asterism also occurs now and then in the "rhodolite" pyrope-almandines from Africa and probably other garnet types that have properly-oriented rutile inclusions. There's a photo of a rhodolite star stone in "Garnet" by John D. Rouse.

Richard M.

 

[Richard M.]

Have any of you seen cat's-eye opalite from Africa? This one is not an especially good example because the eye isn't sharp but it's the best image I could find.

The opalite apparently is a pseudomorph after replacing a fibrous material like asbestos. I've never seen a gemological analysis but it certainly cuts fine cat's-eyes in numerous colors. It cuts very nice stones that at their best look very much like chrysoberyl. I have a small amount of rough in grey, reddish and several shades of yellow to yellow-brown.

Richard M.

 

[Richard M.]

Writing about Idaho and star stones as I did in a previous post, has anyone in the group seen precious opal stars and cat's-eyes? Idaho has both.

An opal deposit near Spencer, ID yields some of the world's finest triplet-making opal. The opal was deposited inside nodules that formed in a perlite flow, no doubt geologically related to nearby Yellowstone National Park. The opal is useful mainly for triplet cutting because absolutely flat thin bands of incredibly bright precious opal are interlayered with white common opal. Now and then a phenomenon unique in the world occurs resulting in star and cat's-eye stones.

Research done in 1975 at the CSIRO Division of Triobophysics, University of Melbourne, Parkville, Victoria, Australia stated that: "Their optical diffraction patterns have been found to contain strong intersecting 'rays' of color confirming that the gems are justifiably called 'star opals'...the packing arrangements which produce them are ...different from those found previously for Australian opals."

These patterns yield single streaks of color at 60-degree intervals as the opal is rotated producing cat's-eye stones after being cut into triplets. Other diffraction patterns yield stars with both three and four legs as triplets. These stones are very rare and not well known.

Richard M.