Why do sapphires change color in different light?

[PrecisionGem]

SOLUX makes very high quality full spectrum bulbs. These are used by art galleries and museums to light up artwork, where you want to ensure you are seeing the true colors. I use these bulbs when photographing the stones as they are very true to natural sunlight.
The worst bulbs for viewing gemstones or really anything are CFL bulbs. They are very strong in the green spectrum, so they do make peridot and chrysoberyl look nice, but for most stones they are horrid.
People will have a hard time with a spectroscope and their gems, if they don’t have a good light source. For a high quality spectroscope set up the light source will often cost almost as much as the scope, as it’s very important to have a full spectrum light with out drastic peaks in certain colors.

 

[Starstruck8]

This is clear, of course, and aligns with what the rest of us are saying. But it does not speak to my brain's ability to color-adjust for everything in the scene except a sapphire or an alexandrite. That's the only place where I disagree with you. I do not think that is the explanation, even partially. My "brain" does not know whether I am looking at a sapphire or a chunk of glass, and the color does not change in my mind when I get close enough to the jewelry case to learn that it is sapphire, say.

For those who are still interested, here is a visual demo.

The comparison below shows photos of a strip of watercolour patches. The top row was shot under an overcast sky and colour balanced from the reference at the right. The bottom row was shot under incandescent (halogen) lights, using the same colour balance. The two pictures have been cut-and-pasted together. Note that everything in the bottom row (including the paper) is yellower. This is no surprise, because the halogen lighting is ‘yellower’.


The next comparison shows the same two shots, but with the halogen shot colour balanced for the halogen lighting. This removes the overall yellow cast. But it still leaves differences. For example, the blue-green patch is bluer. The orange and pinkish patches at the right are a bit lighter and oranger.

Of course, our eyes are not cameras and our brains are not computers, so this demo is only illustrative. But it shows the key points: (a) Our visual system adjusts for changing lighting. If it didn’t, we would see dramatic changes like those in the first comparison. (b) The adjustment is not perfect. It leaves differences, which we see as colour shifts. For most things, these differences are small. But for some things, as for the blue-green patch in this demo, they are larger.

 

[minousbijoux]

@Starstruck8: what a fantastic post and great examples. Thank you. I wish this could be stickied somewhere, perhaps in the newcomers thread on gemstones...

 

[LilAlex]

The bottom row was shot under incandescent (halogen) lights, using the same colour balance

So you mean that you kept the color temperature the same? Not sure what you mean by "color balance." You would need to force the former since most cameras would auto-adjust. You can dial it up or down in post, but the default color temperatures would be different.

Adjusting for color temperature (?), I see almost no difference in the bottom pairing. Would help to show the bottom one as a "reflection" of the top so the same pigment density could be compared side by side. (The marks are not completely uniform.)

Thank you for posting this!

 

[Starstruck8]

So you mean that you kept the color temperature the same? Not sure what you mean by "color balance." You would need to force the former since most cameras would auto-adjust. You can dial it up or down in post, but the default color temperatures would be different.

Adjusting for color temperature (?), I see almost no difference in the bottom pairing. Would help to show the bottom one as a "reflection" of the top so the same pigment density could be compared side by side. (The marks are not completely uniform.)

Thank you for posting this!

Actually, both photos were shot raw (with Nikon D7000), and colour balanced in raw conversion (using Adobe Camera Raw from PS Elements). This allows endless non-destructive rebalancing. The colour balancer has two independent parameters: ‘temperature’ (in effect blue – yellow), and ‘tint’ (in effect green - magenta). You can set these directly, or you can use an eyedropper to click on a reference in the photo – it will then set the parameters to make that reference neutral.

In the first comparsion, I forced the colour balance just as you suspected. For the ‘cloudy’ shot, I used the eyedropper to set the balance from the reference at the right. I then used the same balance parameters for the halogen shot. For the second comparison, I balanced the halogen shot from the reference at the right.

Here is the second comparison, mirrored as you suggested:


The differences are indeed small, except for the blue-green patch. I have a story about that, which I will put in another post. I see differences only for the blue-green patch and for the three at the right.

 

[LilAlex]

The differences are indeed small, except for the blue-green patch. I have a story about that, which I will put in another post. I see differences only for the blue-green patch and for the three at the right.

Sounds like you did it perfectly (which actually means that's how I would have done it ). Nice work! (Probably better than I would have since I forgot that the green/magenta tint slider also moves automatically -- I shoot in RAW and process in Lightroom.)

I really had to stare at that teal one but I (finally) agree with you -- the "top" one is a little greener. And now it seems unmistakable.

 

[Starstruck8]

This is clear, of course, and aligns with what the rest of us are saying. But it does not speak to my brain's ability to color-adjust for everything in the scene except a sapphire or an alexandrite. That's the only place where I disagree with you. I do not think that is the explanation, even partially. My "brain" does not know whether I am looking at a sapphire or a chunk of glass, and the color does not change in my mind when I get close enough to the jewelry case to learn that it is sapphire, say.

So why are some things shiftier than others? What is so special about alexandrite? My earlier posts did not address this, except to say ‘weird spectrum’, which is true but vacuous.

Here is an article explaining the colour change of alexandrite. Explanation of the Colour Change in Alexandrites | Scientific Reports (nature.com)

I have a few reservations about this article. A) It’s not the easiest to follow. Anyone who makes it to the end will understand why I did not try to give a detailed explanation myself. B) The authors seem ungenerous to previous authors. C) They use simple (‘von Kries’) scaling of cone cell responses – this is a good start, but not the last word. D) The story about good adjustment for lighting spectra that are close to black body is plausible but speculative.

That said, it is the best I have been able to find on the net. It specifically does address the question of what is so special about alexandrite. The guts of the story is in Figure 2. This must be read with the paragraph above it – the caption by itself is not enough.

 

[LilAlex]

Here is an article explaining the colour change of alexandrite. Explanation of the Colour Change in Alexandrites | Scientific Reports (nature.com)
_____________________________________________

This is ridiculous. I mean thank you for the link, but it makes no sense. I share your reservations. (This is not my field but I can pretty much follow along.)

There is no way that "color constancy" (aka, the blue-and-black dress meme) accounts for the change in alexandrite color -- we do not see it in other stones under similar conditions and we do not need to know that it is alexandrite for this phenomenon to work.

I was super-impressed to learn in the intro that no one knows why there is the color change -- I mean I do not know but I assumed somebody did. Oh, they cite the chromium stuff -- and then just wave it away.

Nature, the journal (well, the noun, too) is phenomenal. Nature Scientific Reports is meh (impact factor of ~ 4).

And there is this disclaimer with pretty extensive conflicts of interest (not that anyone is getting super rich off Big alexandrite-museum-lighting ):

Competing interests​

E.D.T. owns Absolute Action Ltd which has supplied lighting adapted for alexandrites to the Smithsonian Institution (Washington) and the Natural History Museum (London). D.J.D. acts in an unpaid advisory role to Absolute Action Ltd. A.H., R.H. and J.E.P. are staff of the museums and were involved in the procurement of the lighting from Absolute Action Ltd. C.W.W. donated valuable alexandrites to the Smithsonian, of which she is a member of the Board.

 

[Starstruck8]

Here is an article explaining the colour change of alexandrite. Explanation of the Colour Change in Alexandrites | Scientific Reports (nature.com)
_____________________________________________

This is ridiculous. I mean thank you for the link, but it makes no sense. I share your reservations. (This is not my field but I can pretty much follow along.)

There is no way that "color constancy" (aka, the blue-and-black dress meme) accounts for the change in alexandrite color -- we do not see it in other stones under similar conditions and we do not need to know that it is alexandrite for this phenomenon to work.

I was super-impressed to learn in the intro that no one knows why there is the color change -- I mean I do not know but I assumed somebody did. Oh, they cite the chromium stuff -- and then just wave it away.

Nature, the journal (well, the noun, too) is phenomenal. Nature Scientific Reports is meh (impact factor of ~ 4).

And there is this disclaimer with pretty extensive conflicts of interest (not that anyone is getting super rich off Big alexandrite-museum-lighting ):

Competing interests​

E.D.T. owns Absolute Action Ltd which has supplied lighting adapted for alexandrites to the Smithsonian Institution (Washington) and the Natural History Museum (London). D.J.D. acts in an unpaid advisory role to Absolute Action Ltd. A.H., R.H. and J.E.P. are staff of the museums and were involved in the procurement of the lighting from Absolute Action Ltd. C.W.W. donated valuable alexandrites to the Smithsonian, of which she is a member of the Board.

Well, I should have said that I have a lot of reservations about the article. In fact, I’m surprised that it got published without major changes.

That said… Figure 2 and the paragraph above it give a story explaining why alexandrite shifts, and why the yellow colour they mention does not. Figure 3 shows the effect of varying the cutoff wavelengths of the schematized alexandrite spectrum. It shows that there is only narrow range of cutoffs that give good colour change. So yes, the article does explain what is special about alexandrite.

I was super-impressed to learn in the intro that no one knows why there is the color change -- I mean I do not know but I assumed somebody did.

I had assumed that too, until I started searching. There are standard ‘colour appearance models’ – plug in the spectra, out pops the answer. The problem is, this gives no insight. An explanation (as in, a story that gives insight), is harder to come by. After a fair bit of searching, this article, with all its faults, is the best I have been able to find. If any PSers out there know of a better account, I would really like to hear of it.