Purple and color change

[zeolite]

https://www.pricescope.com/community/threads/would-you-recut-this.110168/

I''m referring to this thread, but wanted to start a new one, since this diverges from the topic of cutting the amethyst.

Any gemstone will appear to change color slightly when viewed in different light. Color change and color shift differ only in the amount of change. Color change is strong, while color shift is weaker.

Both of these terms refer to changes in color of the same gem, viewed in the same direction under different light. Different light often refers to sunlight which is stronger in blue wavelengths, while artificial light, usually incandescent bulbs, are strong in orange and red light.

The dividing line between a normal gem, a color shift gem and a color change gem is not clearly defined. But buyers, who are able to view many gems, especially fine alexandrites, in different types of lighting, will have some idea of what is a color shift gem and what is a color change gem.

FrekeChild: Color change amethyst? Never heard of it.

Cofor: I think the color change is a sales trick. E-bay sellers keep on inventing their own nomenclature to make things more attractive. The "change" is just the ordinary difference in hue of deep colored amethyst, depending on lightning type. Bluish violet in daylight and more reddish in incandescent light.

Cofor is right, this is a marketing trick, exploiting purple and different light sources. All purple gems will be at least a color shift gem (some can be color change). Let me explain the physics:

The color diagram below is the CIE Chromaticity diagram. It defines color in a very exact and repeatable way. Physicists use a color instrument, called a spectrophotometer, which can measure the wavelength of light in extremely tiny (nanometer) wavelength increments. If the color is very pastel, it will be plotted closer to the center of this diagram. If the color is very pure and saturated, it will be plotted inside the diagram, but very close to the outline of the two curved sides of this diagram. The intensity of fine ruby, sapphire, and emerald is very high and their color will be plotted very close to the outline of the diagram. The one straight side is a forbidden zone, which I''ll explain below.

Please bear with me on my names of colors; it is required for the physics below. At the bottom of the diagram, the shortest wavelength our eye can see is violet. As we travel to the upper left, we plot blue, then at the upper left curved point, green, then yellow, orange, and red. The importance of this curve is that each color can be defined by one single wavelength.

Guess what color is not on the curve? Purple. In the physical sense, purple is not a color. That is, it can''t be represented by one wavelength. All of the colors around the two curved sides of the triangle can be created by one wavelength.

So where is purple on this diagram? It is in the middle of the one straight side of the diagram (the forbidden zone), because purple is made up of both red and blue wavelengths.

So how does this relate to purple gems and color change? Daylight is relatively strong in blue wavelengths. Incandescent light is strong on red. Purple gems viewed in daylight will appear to the blue side of purple (violet). When viewed in incandescent lighting, the same gem will appear to the red side of purple (magenta).

So all purple gems (including amethyst, sapphire and especially tanzanite) will appear to have some amount of color shift. Yes, color change amethyst is a marketing trick.

 

[mercoledi]

Really cool post, thanks.

ETA: What's up with alexandrite then? It does have a purpleish color, but the 100% changers go from green to red. What's the physics behind that?

 

[zeolite]

Keep in mind that purple is the only color made from two (widely spaced) wavelengths.

The physics of all other color change gems is this: All color change stones have a broad absorption band somewhere in the yellow region, which normal gems do not have. This band is caused by as vanadium and/or other trace metals. The position of the band (green, yellow-green, yellow, orange) determines the two different colors that are shown (red - green, red - blue, peridot green - orange, etc).

The absorption band blocks mid-wavelength colors (yellow-green, yellow, and orange). If you view a color change gem in daylight, there is little red, yellow is blocked, so the gem shows cool colors (green or blue). If you view that same gem in incandescent light, there is little blue, and yellow is blocked, so the gem shows warm colors (orange or red).

The cause of the color change is the width and position of the mid-wavelength absorption band.

 

[Lady_Disdain]

Thank you for that explanation.

 

[T L]

I can tell you''re a physicist.

Do gems have a doppler effect as well? Ha!!

 

[ma re]

I read once that a color shift happens when (for example) gem goes from yellow to greenish yellow, and that color change is when it goes from yellow to green (changes for a whole hue). Not much of an explanation, but probably better than nothing

 

[zeolite]

Date: 3/18/2009 11:06:03 PM
Author: tourmaline_lover
I can tell you''re a physicist.

Do gems have a doppler effect as well? Ha!!

Now that''s a thought: increase the value of your gem collection through the doppler shift. Rubies of equal size and quality are more expensive than blue sapphires. So you just accelerate your blue sapphire away from you, to nearly the speed of light, and it appears as red as a ruby!

The minor problem with that is, it is difficult to view and enjoy it, when it is traveling 185,000 miles per second, and the energy cost of getting to that speed is excessive.

 

[zeolite]

Date: 3/19/2009 4:45:44 AM
Author: ma re
I read once that a color shift happens when (for example) gem goes from yellow to greenish yellow, and that color change is when it goes from yellow to green (changes for a whole hue). Not much of an explanation, but probably better than nothing

Most people, without access to a spectrophotometer, would assume that there is even and equal spacing (of wavelength) between all colors. That is not at all true.

The smallest spacing is yellow, at about 10 nanometers (nm) of wavelength. 565 nm is yellow-green, 575 nm is yellow, and 580 nm is golden. So you pass completely through yellow in about 10 nm. The broadest color is red, from 630 nm to 750 nm. That says you can travel 12 times the width of yellow and yet still remain within the color red!

All this says that if you want a color change to be through an entire hue, it is easiest to accomplish when one end is in yellow.

 

[FrekeChild]

Very cool explanation. I thought that a color change amethyst sounded fishy. It seems logical to me that most gemstones would have a slight color change depending on the wavelengths and color of the light source. I have a medium purple amethyst, and it looks more blue-purple outside and more pink/red-purple inside.

Anyway, its very interesting...

 

[T L]

Date: 3/19/2009 1:09:32 PM
Author: zeolite

Date: 3/18/2009 11:06:03 PM
Author: tourmaline_lover
I can tell you''re a physicist.

Do gems have a doppler effect as well? Ha!!

Now that''s a thought: increase the value of your gem collection through the doppler shift. Rubies of equal size and quality are more expensive than blue sapphires. So you just accelerate your blue sapphire away from you, to nearly the speed of light, and it appears as red as a ruby!

The minor problem with that is, it is difficult to view and enjoy it, when it is traveling 185,000 miles per second, and the energy cost of getting to that speed is excessive.

Since your mass must approach infinity to get to the speed of light, it would be a pretty BIG stone. Ha! Where''s Einstein when you need him!?

 

[chrono]

I read that a colour shift is when it moves 1 step in the colour spectrum, meaning green to yellow. A colour change is when it moves to the opposite end of the colour wheel, as in green to red.

 

[zeolite]

I just looked up my GIA course work, under alexandrite. It says:

"Chrysoberyl that exhibits a distinct change of color from red under candlelight or tungsten-filament incadescent bulbs to green in daylight or fluorescent illumination is called alexandrite. The name is only applicable when a distinct color change is visible."

There is no mention or discussion of color shift gems.