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'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)
  
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Amir Akhavan




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PostPosted: Jan 03, 2010 14:06    Post subject: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

When I visited the Tokyo Mineral Show in December 2009, I noticed 2 dealers that offered quartz specimens that I have never seen before, including the Munich show the month before (and I was at the same dealers booth in Munich).

First I thought "Woaahh - they really offer tons of lame stuff here for insane prices. Does this sell?".
(see first image and note the Yen per gram price)

But than I had a closer look.



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rq_1.jpg



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PostPosted: Jan 03, 2010 14:11    Post subject: Re: 'Rainbow Quartz' ... An Old Novelty (or What I'd expect to see in Tucso  

The material is sold as "rainbow quartz" and comes from some locality in Madhya Pradesh, India.
The dealer I got my specimen from wrote down "Karvy, Madhya Pradesh", but I can't find a village or town with that name anywhere.

I just write what will soon appear like this or slightly modified on my own website:

================================
"The other type of iridescence is much rarer and has first been described by C.V. Raman in 1950. Here the cause of the color is not a crack, but a diffraction grating very similar to the effect seen in rainbow or iris agate. While in iris agate the grating is made of layers of different refractive indices, in rainbow quartz it is probably caused by polysynthetical Brazil law twin layers. This type of twinning is very common in amethysts, where the individual twin layers are usually large enough to be seen in a thin section under polarized light. They may get very thin though, and have been the proposed reason for the observed color play in certain crystals from Uruguay and India. The Indian specimens are ususally colorless but resemble the typical amethysts from volcanic gas cavities in any other respect, and some of the specimen I have seen do have a faint amethyst color. The image shows such a specimen from Madhya Pradesh, India. The effect is very subtle in most specimen and while it is easily seen on a specimen that is turned in your hand, it is very difficult to capture on a photo. Individual rhombohedral faces reflect light in a colorful way, most often in green and pink, but the color does not change with the angle of the incident light: a rhombohedral face that looks green always looks green, no matter how one turns the crystal, a pink face looks always pink. A few crystal faces show color transitions, from blue to pink, for example, which also do not change when turning the crystal. Sometimes colored reflections are only seen on parts of a crystal face. In general only a few faces of a specimen show the effect strongly, many faces do not show the effect at all. According to Raman (1950), the reflected light is almost monochromatic, which is a strong indication of a diffraction grating. On the other hand it is difficult to understand how pink reflections can be caused by monochromatic light. The colored reflections are not at the surface, but come from within the crystal and mix with the regular white reflexction from the surface, which gives them a shine that is not unlike but much weaker than that of the infamous aqua aura gold-covered bluish quartz crystals."
================================

That's what I got so far.
I do not yet fully understand how a Brazil law twinning alone would cause such an effect without being accompanied by some minor cracks between the layers as there is no difference in refractive indices, and there seems to be some misty bluish zone about where the reflections originate.
Maybe I read the Raman paper again.

Here's where I would like to put the reference for the Raman paper and other thinks but the forum software thinks this is spam (it is right ;-) )

So what is the trick?

Finally a photo of the specimen I got. It is 63mm wide.



q-rainbow_in_karvy_Q505_1_sml.jpg
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Rainbow Quartz, 63mm
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q-rainbow_in_karvy_Q505_1_sml.jpg



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PostPosted: Jan 03, 2010 14:16    Post subject: Re: 'Rainbow Quartz' ... An Old Novelty (or What I'd expect to see in Tucso  

Finally the NGL Certificate that I unfortunately can't read because it is written in Japanese.
This should resolve all your doubts ;-)

There's apparently a 1988 Gems and Gemology paper discussing the subject.

Cheers
Amir



rq_2.jpg
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Nippon Gemstone Laboratory Certificate
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rq_2.jpg



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PostPosted: Jan 03, 2010 14:54    Post subject: Re: 'Rainbow Quartz' ... An Old Novelty (or What I'd expect to see in Tucso  

Amir Akhavan wrote:
Here's where I would like to put the reference for the Raman paper and other thinks but the forum software thinks this is spam (it is right ;-) )
So what is the trick?

Amir, please read -> http://www.mineral-forum.com/message-board/viewtopic.php?t=74

Jordi
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PostPosted: Jan 03, 2010 15:24    Post subject: Re: 'Rainbow Quartz' ... An Old Novelty (or What I'd expect to see in Tucs  

Jordi, thanks!

So these are the references:

www(dot)coloradogem(dot)com(slash)index(dot)php?display=adularescence
members(dot)cox(dot)net(slash)cgmaz(slash)

http://www.ias.ac.in/j_archive/proca/31/5/275-279/viewpage.html
(link normalized by FMF)

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PostPosted: Jan 03, 2010 15:27    Post subject: Re: 'Rainbow Quartz' ... An Old Novelty (or What I'd expect to see in Tucs  

sigh...
good luck the references are still human-readable ;-)

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PostPosted: Jan 03, 2010 16:09    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

Amir Akhavan wrote:
...the references are still human-readable ;-)

Thanks Amir. I already normalized the second link, not the first one because is sending to a commercial web page...

Jordi
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PostPosted: Jan 04, 2010 12:18    Post subject: Re: 'Rainbow Quartz' ... An Old Novelty (or What I'd expect to see in Tucso  

Amir Akhavan wrote:
There's apparently a 1988 Gems and Gemology paper discussing the subject.


Hi Amir,

The G&G reference is page 250 of Winter 1988 and an earlier one on page 240 of Winter 1987. The latter describes the "Lowell" effect seen in amethyst from Artigas, Uruguay which was confined to the minor rhombohedral faces and areas immediately surrounding them; this was the first time those at the lab had seen the phenomenon (editor John Koivula). The later 1988 reference noted a very similar effect in colorless quartz (as sections of geodes) from near Poona, India. They were determined not to be thin film effects, but rather diffraction produced colors, again confined to the minor rhombohedral faces (editors John Koivula and Robert Kammerling).

In John's Vol 2 PhotoAtlas 2005, he again describes these two quartz - in the Artigas amethyst: Lowell effect, apparently "related to lamellar twinning in the outer layers of the minor rhombohedral faces...sometimes so prominent that it enhances the body color..." (p. 568). In Orissa India quartz, spectral interference colors in the rhombehdral plane under the rhombehdral face (p..569 - I am not clear if this is related). Page 655 also shows a colorless quartz from Washington State exhibiting the "Lowell Effect."
(Gübelin, Eduard J. and John I Koivula. (2005) Photoatlas of Inclusions in Gemstones, Volume 2. Basel: Opinio Publishers. 829 pages).

I brought these up when I saw your posting b/c there seems to be some new thought on the mechanism in the intervening 17 years, but I don't find references in the meantime. You might contact John as quartz and its inclusions is a great love of his; I believe I have pointed him to your wonderful website in the past at one time or another. I had run across this "Lowell Effect" a while ago during one of my literature searches, but had forgotten about it until you posted these interesting pictures. The C.V. Raman paper is a very welcome addition; thank you for posting it.

Elise

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PostPosted: May 17, 2010 16:36    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

Hi,

a friend of Alfredo, Yuko Tanaka, pointed out to me that the colors of the internal reflections do change in some of her specimens, and provided some photos, including some of cut stones.
I recently bought 2 diffraction filters for optical purposes (microscopy) and learned that the color (reflected and transmitted) is very much dependent on the angle of incidence of the light.

So Yuko was right and I was wrong, and I have to correct what I wrote in my earlier posting. I will simply copy what is written (corrected) on my website now:

---------------------------
" ... The effect is very subtle in most specimen and while it is easily seen on a specimen that is turned in your hand, it is very difficult to capture on a photo. The colored reflections are not at the surface, but come from within the crystal and mix with the regular white reflection from the surface, which gives them a shine that is not unlike but much weaker than that of the infamous aqua aura gold-covered bluish quartz crystals: zones immediately under the rhombohedral faces reflect light in a colorful way, most often in green and pink. Sometimes colored reflections are only seen on parts of a crystal face. In general only a few faces of a specimen show the effect strongly, many faces do not show the effect at all.

The color normally does not change with the angle of the incident light: a rhombohedral face that looks green always looks green, no matter how one turns the crystal, a pink face looks always pink. I've seen photos[3] of a specimen with faces that show different colors depending on the angle of the incident light at the same face - and this is actually the expected behavior of a diffraction grating, which filters and reflects light of different wavelengths depending on the angle of the incident light. A possible explanation: the intensity of reflections at the surface depends on the angle, once the angle of incidence falls under a certain value, the reflection is total, and no light will enter the crystal. Since the diffraction grating is usually parallel to the crystal surface, its reflections get the weaker the more shallow the angle of the incident light is. On the other hand, if the light enters the crystal roughly perpendicular to the surface, the reflections from the surface and the underlying grating are both very weak, and most light passes through. So there's only a narrow range of angles that will cause strong reflections at the grating and the color of that reflection does not change very much. This, of course, is just an untested proposal to bring the observations in line with the theory. According to Raman (1950), the light transmitted through the reflcetive patches is almost monochromatic, which is a strong indication of a diffraction grating. Pink reflections can be caused by monochromatic light. Cut stones seem to show color changes of the reflections more frequently[4], which would be expected, because the stone's surface is usually not parallel to the reflective zone inside. "

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

Sorry for keeping silent on the forum, but I've been too busy the past months, and this will go on for some time.

Cheers
Amir

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PostPosted: May 17, 2010 18:12    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

Those of us who use Facebook can see some more pictures of this weird effect on the "Colorstone Mithrill" page.
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PostPosted: May 22, 2010 21:19    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

Just curious, why did Elise keep referencing the "Lowell effect" in quotations? Is this not a valid name for the phenomenon?

- Tracy

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PostPosted: May 26, 2010 12:45    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

Tracy wrote:
Just curious, why did Elise keep referencing the "Lowell effect" in quotations? Is this not a valid name for the phenomenon?- Tracy

Hi - I am not sure how a term used in one field of study (gemology in my case) gets to be "official" in another specialty, let alone in other languages. To the best of my knowledge, this term came about as a result of the first report in the gemological literature where it was written that Jack Lowell had observed what was thought to be a previously unreported phenomenon (if it is the same effect, then the 1950 paper by Sir Raman was overlooked). It was dubbed 'Lowell" effect by John Koivula in that entry and subsequent ones, as well as in his Photoatlas books. I will send this link and the Raman paper to John and see if he comments; especially on the angle. As I wrote previously, quartz and its inclusions are his special interest. There is a picture of spectacular color in a colorless quartz on page 644 of his Photoatlas Vol. 2 which is much more dramatic than the effect seen in all the other images (that was a typo above when I wrote 655).

Cheers,
Elise

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PostPosted: Nov 03, 2011 16:06    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

In case anyone is still interested in this old topic, I have collaborated with a Japanese collector to write up some of the historical background:

http://www.mindat.org/article.php/1335/Iris+Quartz
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PostPosted: Nov 04, 2011 01:36    Post subject: Re: 'Rainbow Quartz' ... An Old Novelty (or What I'd expect to see in Tucso  

Amir Akhavan wrote:

I just write what will soon appear like this or slightly modified on my own website:

================================
"The other type of iridescence is much rarer and has first been described by C.V. Raman in 1950. Here the cause of the color is not a crack, but a diffraction grating very similar to the effect seen in rainbow or iris agate. While in iris agate the grating is made of layers of different refractive indices, in rainbow quartz it is probably caused by polysynthetical Brazil law twin layers. This type of twinning is very common in amethysts, where the individual twin layers are usually large enough to be seen in a thin section under polarized light. They may get very thin though, and have been the proposed reason for the observed color play in certain crystals from Uruguay and India. The Indian specimens are ususally colorless but resemble the typical amethysts from volcanic gas cavities in any other respect, and some of the specimen I have seen do have a faint amethyst color. The image shows such a specimen from Madhya Pradesh, India. The effect is very subtle in most specimen and while it is easily seen on a specimen that is turned in your hand, it is very difficult to capture on a photo. Individual rhombohedral faces reflect light in a colorful way, most often in green and pink, but the color does not change with the angle of the incident light: a rhombohedral face that looks green always looks green, no matter how one turns the crystal, a pink face looks always pink. A few crystal faces show color transitions, from blue to pink, for example, which also do not change when turning the crystal. Sometimes colored reflections are only seen on parts of a crystal face. In general only a few faces of a specimen show the effect strongly, many faces do not show the effect at all. According to Raman (1950), the reflected light is almost monochromatic, which is a strong indication of a diffraction grating. On the other hand it is difficult to understand how pink reflections can be caused by monochromatic light. The colored reflections are not at the surface, but come from within the crystal and mix with the regular white reflexction from the surface, which gives them a shine that is not unlike but much weaker than that of the infamous aqua aura gold-covered bluish quartz crystals."
================================


Hi, i'm novice in the internet application. Watching discussions on the forum for about 1 month, now I've got idea that can show up something interesting from Kyrgyz Republic, FSU.
Specimen attached was found in the mountains in skarn zone, possibly with superposed alteration. Each face of all garnet crystals shows that rainbow effect, strong on big faces and a little bit minor on small one. Color varies from blue to green and yellowish. No effect is observed on the broken face. It is not a film (not been removed mechanically). Actually, effect is more expressed than on the photo.

Thanks for any comments.

Alex Chaus



01115.JPG
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Garnet with rainbow effect in actinolite
Kyrgyzstan, FSU
10 x 6 cm, main crystal 10 x8 mm
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01115.JPG


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PostPosted: Nov 04, 2011 07:16    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

Alex,
Iridescent garnets have been produced commercially in Mexico, and from New Mexico, and from Nara prefecture in Japan. If you google "rainbow garnet", you'll find lots of sites with information.

The cause of the iridescence in quartz and garnet is the diffraction grating mechanism in both cases, but with a structural difference: extremely narrow twinning lamellae in the case of the quartz, and extremely narrow growth zoning in the case of garnets.
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PostPosted: Nov 08, 2011 00:11    Post subject: Re: 'Rainbow Quartz'...An Old Novelty (or What I'd expect to see in Tucson)  

alfredo wrote:
Alex,
Iridescent garnets have been produced commercially in Mexico, and from New Mexico, and from Nara prefecture in Japan. If you google "rainbow garnet", you'll find lots of sites with information.

The cause of the iridescence in quartz and garnet is the diffraction grating mechanism in both cases, but with a structural difference: extremely narrow twinning lamellae in the case of the quartz, and extremely narrow growth zoning in the case of garnets.
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