FMF - Friends of Minerals Forum, discussion and message board

[Amir Akhavan]

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.

[Amir Akhavan]

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."
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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.

[Amir Akhavan]

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

[Jordi Fabre]

[Amir Akhavan]

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)

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

[Amir Akhavan]

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

[Jordi Fabre]

[Elise]

[Amir Akhavan]

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. "

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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

[alfredo]

Those of us who use Facebook can see some more pictures of this weird effect on the "Colorstone Mithrill" page.

[Tracy]

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

- Tracy

[Elise]

[alfredo]

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:

https://www.mindat.org/article.php/1335/Iris+Quartz

[alex chaus]

[alfredo]

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.

[alex chaus]