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User gallery only Iris quartz

Posted by Jason Evans  
Jason Evans June 29, 2017 06:33PM
I am curious why my Iris quartz photos have been placed in user gallery only, I took quite a bit of time experimenting with the best lighting to show the colours,and I don't think there is anything wrong with the quality of the photo's, I don't think there is anything lacking in the info I have provided is there some other reason

Edited 1 time(s). Last edit at 06/29/2017 06:34PM by Jason Evans.
Alfredo Petrov June 29, 2017 06:45PM
Thanks, Jason. Back in public view.
Jason Evans June 29, 2017 06:58PM
Thank you, Alfredo!
Wayne Corwin June 29, 2017 08:37PM
Nice one Jason !
Good job !
Alfredo Petrov June 29, 2017 08:51PM
I admire Jason's tenacity in getting the shot, because my own attempts to photograph iris quartz were completely unsuccessful. The material lends itself better to video, where you can watch the colors while rotating it. Hard to capture much with a still shot.
Wayne Corwin June 29, 2017 09:18PM
It should qualify for a POTD !
Steven Kuitems July 11, 2017 05:25AM
Agree awayne, great shot. The iris-refraction is from Brazil law twining planes on these crystals.
Matt Ciranni July 11, 2017 07:39PM
Are these treated, or natural?
I've seen a few crystals like this in rock shops, but I was told they were coated with a substance to give that iris iridescence.
Alfredo Petrov July 11, 2017 08:02PM
Completely natural, Matt. The iridescence is on the inside of the quartz crystals. And yes there are rock shops selling coated quartz with the iridescence produced by an artificial surface coating, but it's easy to tell the difference.

Rob Woodside July 12, 2017 05:57AM
The interference colours, at least on my piece, are from the parting on the rhombohedral faces which permits a gap that acts like a thin film. Pressure on the rhombohedral face shrinks the gap and the colour changes appropriately. Similarly, submersed in water the refractive index of the gap increases and again the colour changes appropriately.
Steven Kuitems July 13, 2017 02:00AM
Check out the Quartz page web site and see their discussion on Brazil law twinning, when the planes line up with external faces and just below their surface the layer gap refracts the white light.
Rob Woodside July 13, 2017 03:54PM
Thanks Steven. The way that is written it sounds like there are alternating layers of left and right quartz in the r faces that could diffract the light into its spectra. If that was the case the colour would not change with pressure or on immersion. This is a thin film interference effect and not a diffraction phenomenon. These twin layers must promote the parting to give the necessary air gap.
Alfredo Petrov July 13, 2017 04:47PM
We are all a bit out of date... Gems & Gemology recently published a detailed study of Iris Quartz: https://www.gia.edu/gems-gemology/spring-2017-iridescence-natural-quartz
Jason Evans July 13, 2017 10:22PM
That was a very interesting read, thanks for posting Alfredo. I have to confess some of it I did not understand but I think I got the basic gist It is good to see that new and vigorous scientific studies have been made to find out the cause of the iridescence.
I have just been examining my specimen with my microscope, it only has 20x magnification but I can see lamelar features, it's quite striking they look like tiny laser beams inside the crystal.
If I have understood it, or if i have misinterpreted what Rob said about his specimen these new findings appear to contradict Robs findings.
Thanks also the for kind comments about my photo, it is always nice when people appreciate the effort, especially when they know from experience that it is tricky to capture the iridescence on a still image.
Rob Woodside July 17, 2017 06:41PM
Well, What to say? They immediately give a good explanation of thin film interference and diffraction. Then they dismiss thin film interference as that would depend on gap thickness and refractive index of the gap. This is precisely what I found. Putting pressure on the face shrinks the gap and changes the colour. Immersed in water overnight changes the refractive index of the gap and the colour.

The array of high tech equipment brought to bear on this is quite impressive. Much of it looks at the surface which even Raman said was unimportant. Around fig 14 you can see separations inside the quartz which could be parting. In spite of their impressive technology they can't account for simple observations..
Uwe Kolitsch September 27, 2017 01:51PM
See also recent publication:
Xiayang Lin and Peter J. Heaney (2017): Causes of iridescence in natural quartz. Gems & Gemology, 53, 68-81.
(Added to quartz and iris quartz pages.)
John Montgomery September 27, 2017 03:30PM
Would this specimen qualify as "iris quartz"? Thank you
Jason Evans September 27, 2017 11:26PM
I would not call it Iris quartz, I can see rainbow colours but I think they are from inclusions of haematite.
John Montgomery September 28, 2017 02:24AM
Thanks for responding Jason. Rob Woodside made the same conclusion in personal correspondence.
Rob Woodside September 28, 2017 04:54PM
With an Iris quartz and a tunable laser one could check this published diffraction theory.

With a known fixed wavelength one could look for higher order diffraction peaks. That alone would prove the diffraction theory. A laser's intensity is needed to observe higher order peaks. (The fast Fourier Transform shown in Figure 13 in the Lin Heaney paper [thanks for the link Uwe] certainly hints at crude diffraction)

Using a fixed wavelength and the diffraction angles, one could have an independent measure of the "grating" spacing. This could then be used to predict the angles of diffraction peaks at different wavelengths and give a further check.Getting agreement between the published high tech grating and that found with a high intensity laser would be impressive..

It could be that both diffraction and thin film interference are at work here, but my simple observation of changing the colour by applying pressure shows that thin films are involved. Lin and Heaney say "The absence of either opal or compositional impurities definitively rules out iridescence by a thin-film effect, sensu stricto." and that is simply wrong. In xls with internal fractures there can be no compositional impurities and no opal like structures, but one does see the thin film interference colours in the fractures.

Edited 2 time(s). Last edit at 09/28/2017 05:38PM by Rob Woodside.
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