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Alexandrite color change how many Kelvin to measure

Jollyroger

Rough_Rock
Joined
Sep 28, 2020
Messages
7
Hello, guys! I've just bought 2 Indian alexandrites. I'm so confused about measuring the color change. Because the incandescent light can be different if to measure it Kelvin, does it mean the stones have to exhibit different color? So my stones mist of the time are emerald green (bluish green) - picture 1. Even in the evening outside I see it. If I'm with some incandescent or yelowish light (around 2500-3000 Kelvin?) they are mostly blue-purple to the eye(picture 2) The biggest color change I see if they are in the candle light picture3). Here is an amethyst color with red flashes, depends on the angle. So the question is, what changed color is true? Bluish or amethyst? When they measured it in 19 century, there was not any light but candle. If the stones exhibit strong change to amethyst color with candle light(1500 Kelvin?) and strong change to purple-blue in tungsten lamp, but most of the time they remain blue-green, does it mean they have weak color change? Stones with weak color change can change completely the color or not? Thank you very much!
 

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Bron357

Ideal_Rock
Premium
Joined
Jan 22, 2014
Messages
4,605
What affects the “colour change” in Alexandrite is the wavelength of the light source. incandescent light has a longer wavelength than daylight. Under an incandescent light source the Shorter wave length colours ie green and blue are subdued and the longer orange and red are more dominant.
When a gem lab assesses the colour change in an Alexandrite they use an incandescent light source around 3300K.This is most similar to the light created by a candle. Daylight wave length is around 5100K. More often “photographed” Alexandrites images are adjusted because cameras don’t necessarily match what the eye “sees” and incandescent shots in a studio / light box are controlled. Mixed lighting, combination of longer and shorter wave lengths often turns the colour to “meh”.
In “real life” there are few incandescent lights these days, they aren’t energy efficient and are “hot” so most lights are now are fluorescent or LED. These don’t create the same colour change effect.
Alexandrite colour change is assessed two way.
Firstly the actual colours of the gem, bluish green in daylight to purple red in incandescent are considered ideal. Any shift to yellowish tone in daylight or brownish or grey tone under incandescent is considered less desirable.
Secondly is the % and strength of the colour change. how much of the gem changes colour and how intense is that change. Again the more exaggerated the change the ”better” the Alexandrite.
The reality of having an Alexandrite is that you most often see the daylight colour and that can be variable depending on where you live and the season as both these factors change the wavelength of daylight. And unless you have a supply of candles or have old fashioned incandescent light bulbs, you’ll rarely see the purple red.
 

Jollyroger

Rough_Rock
Joined
Sep 28, 2020
Messages
7
What affects the “colour change” in Alexandrite is the wavelength of the light source. incandescent light has a longer wavelength than daylight. Under an incandescent light source the Shorter wave length colours ie green and blue are subdued and the longer orange and red are more dominant.
When a gem lab assesses the colour change in an Alexandrite they use an incandescent light source around 3300K.This is most similar to the light created by a candle. Daylight wave length is around 5100K. More often “photographed” Alexandrites images are adjusted because cameras don’t necessarily match what the eye “sees” and incandescent shots in a studio / light box are controlled. Mixed lighting, combination of longer and shorter wave lengths often turns the colour to “meh”.
In “real life” there are few incandescent lights these days, they aren’t energy efficient and are “hot” so most lights are now are fluorescent or LED. These don’t create the same colour change effect.
Alexandrite colour change is assessed two way.
Firstly the actual colours of the gem, bluish green in daylight to purple red in incandescent are considered ideal. Any shift to yellowish tone in daylight or brownish or grey tone under incandescent is considered less desirable.
Secondly is the % and strength of the colour change. how much of the gem changes colour and how intense is that change. Again the more exaggerated the change the ”better” the Alexandrite.
The reality of having an Alexandrite is that you most often see the daylight colour and that can be variable depending on where you live and the season as both these factors change the wavelength of daylight. And unless you have a supply of candles or have old fashioned incandescent light bulbs, you’ll rarely see the purple red.
Thank you for the great answer! Does it also mean, the better color change is, stronger is pleochroism and more colors together you see in the mixed light?
Also what we understand by the mixed light? White and incandescent? I've seen Brazilian alex in the office white light. The stone's color was weird, like you mentioned above. For me it looked like a greenish-blueish--grayish sapphire, the color wasn't beautiful by my opinion. But it exhibited nice purplish red under the incandescent flashlight (not the flame)
Also the stone has a beautiful emerald color by day, but doesn't exhibit a strong pleochroism, and looks grayish under incandescent light (not the flame), which stone is more desirable?
Thank you!!
 

Bron357

Ideal_Rock
Premium
Joined
Jan 22, 2014
Messages
4,605
Pleochrorism is different to colour changing per see. A number gemstones display a different colour / tone depending on which optical axis is viewed. This is why gem cutters for say sapphire have to consider a number of aspects before cutting, which orientation gives the best colour vs the negatives that might also then affect the sapphire crystal ie smaller end result or more noticable colour zoning.
Alexandrite are both pleochroic and colour change, colour change still occurs Irrespective of the optical axis chosen.
Alexandrite shows colour change because at the distinctly different light wave lengths - daylight vs incandescent - the crystal absorbs / reflects different ends of the white light spectrum before return “to the eye”. Mixed lighting is “in the middle” it is part daylight level (green blue) and part incandescent level (purple red) and the combination of both light frequencies, like when you mix green blue paint together with red purple paint, gives you a rather ugly and “flat” greyish brownish colour.
Alexandrite often looks “better” under either daylight or incandescent light and it’s usually a personal decision as to which gem to choose in such cases. Many would argue that daylight colour is more important because that’s what you’ll see more of, especially as incandescent light sources aren’t plentiful.
My advice is always buy what you love. 7FED6D70-34CB-4822-B65D-EACF21A63669.jpeg F991E60B-C2E8-4EDE-BEA1-429F96FE7721.jpeg
 

Bron357

Ideal_Rock
Premium
Joined
Jan 22, 2014
Messages
4,605
And I will add be sure to see an appropriate an lab certificate before purchasing an Alexandrite. As with many valuable gemstones there are lab grown look a likes out there and you don’t want to pay the price for a rare and expensive gem if it’s not natural.
 

Jollyroger

Rough_Rock
Joined
Sep 28, 2020
Messages
7
And I will add be sure to see an appropriate an lab certificate before purchasing an Alexandrite. As with many valuable gemstones there are lab grown look a likes out there and you don’t want to pay the price for a rare and expensive gem if it’s not natural.
Thank you again for the professional and interesting answer!
Your alexandrite looks gorgeous! Full of life and amazing color change! I wonder what is the origin
 

Nick_G

Shiny_Rock
Joined
Sep 7, 2018
Messages
163
My best alexandrite, which is a mineral specimen, looks a lovely deep emerald green in daylight, but a drab dark greenish-grey in old-school incandescent light, with a small area showing a hint of purple. However, in candlelight or firelight it does go a deep garnet-like red. I took it to a gem show, and in the torches that dealers use to evaluate alexandrite it did look a purple colour. This is the best picture I've managed to take, but even then it's not showing the full green colour:
691379
 

Jollyroger

Rough_Rock
Joined
Sep 28, 2020
Messages
7
My best alexandrite, which is a mineral specimen, looks a lovely deep emerald green in daylight, but a drab dark greenish-grey in old-school incandescent light, with a small area showing a hint of purple. However, in candlelight or firelight it does go a deep garnet-like red. I took it to a gem show, and in the torches that dealers use to evaluate alexandrite it did look a purple colour. This is the best picture I've managed to take, but even then it's not showing the full green colour:
Russian alexandrite specimen.JPG
Where is this specimen from? And what the size is? It looks beautiful! And what did you mean by old school incandescent light? Tungsten lamps? If yes, does it mean the dealers use longer waves light (more yellow) torches than classic tungsten lamps give us, and it affects the color change more?
Thank you!
 

Nick_G

Shiny_Rock
Joined
Sep 7, 2018
Messages
163
Where is this specimen from? And what the size is? It looks beautiful! And what did you mean by old school incandescent light? Tungsten lamps? If yes, does it mean the dealers use longer waves light (more yellow) torches than classic tungsten lamps give us, and it affects the color change more?
Thank you!
Hi Jollyroger. Well, I'm not entirely sure where this specimen is from, because believe it or not it was sold as an epidote from Långban, Sweden, via Wards dating to 1934! There is a very old $8.00 price sticker on the base, which, after finding another specimen with the exact same style of label from another dealer would indeed appear to confirm it was via Wards, but dating to the 1890s. The most plausible theory is that it was acquired by Swedish mineralogist Gustav Flink (who specialised in Långban minerals) on a trip to Russia and it got sent to Wards in the US, and labels/info got confused over the years. I emailed the Natural History Museum in Stockholm with this idea and they agreed. The 1890s label date would also fit with this timeline, as Flink's Russia trips took place between 1889 and 1916.

Size: about 35 mm by 35 mm by 25 mm, and the weight is about 42 grams. The incandescent light I referred to is indeed tungsten lamps. We still use them here. And yes, I presume the torches that the dealers use must be closer to candlelight.
 

Skyjems

Shiny_Rock
Trade
Joined
Jul 9, 2020
Messages
123
SO MANY THINGS TO WRITE

I may come back because I have a busy day, but I wanted to clear up a few things.

1. Colour Change is usually tested around 2800-3300k and 5000-5500k
2. 'daylight' = temperature of the sun at noon, the colour temperature of the sun ranges from about 2000k at sunrise to about 5500k at it's daily peak, to about 3300k at sunset.

THIS MEANS: Your alexandrite should change colour throughout the day, the best alex show some purple/red flash from the right angles even when the sun is at it's apex.

3.
In “real life” there are few incandescent lights these days, they aren’t energy efficient and are “hot” so most lights are now are fluorescent or LED. These don’t create the same colour change effect.
This is only partly true, although incandescent lights are mostly a thing of the past, most modern LED lights have a 'temperature selection' on them, most commonly: 3000k/4000k/5000k

If they don't have this feature, the kelvin is noted on the box and you can buy warm or cool LED lighting, a 3000k LED changes the colour of a gem just as well as a 3000k incandescent light.

4. 'What lighting to use?': this is the standard, just don't put the light right up to the gem, you need to shine it from about 3 feet away to get the proper colour: https://www.aajewelry.com/51-955.html
 

Jollyroger

Rough_Rock
Joined
Sep 28, 2020
Messages
7
Hi Jollyroger. Well, I'm not entirely sure where this specimen is from, because believe it or not it was sold as an epidote from Långban, Sweden, via Wards dating to 1934! There is a very old $8.00 price sticker on the base, which, after finding another specimen with the exact same style of label from another dealer would indeed appear to confirm it was via Wards, but dating to the 1890s. The most plausible theory is that it was acquired by Swedish mineralogist Gustav Flink (who specialised in Långban minerals) on a trip to Russia and it got sent to Wards in the US, and labels/info got confused over the years. I emailed the Natural History Museum in Stockholm with this idea and they agreed. The 1890s label date would also fit with this timeline, as Flink's Russia trips took place between 1889 and 1916.

Size: about 35 mm by 35 mm by 25 mm, and the weight is about 42 grams. The incandescent light I referred to is indeed tungsten lamps. We still use them here. And yes, I presume the torches that the dealers use must be closer to candlelight.
Hi, Nick! This is such an interesting story and great investigation! The specimen looks Russian, this beautiful pine green color! And the size is great, museum piece:love:
 

Jollyroger

Rough_Rock
Joined
Sep 28, 2020
Messages
7
SO MANY THINGS TO WRITE

I may come back because I have a busy day, but I wanted to clear up a few things.

1. Colour Change is usually tested around 2800-3300k and 5000-5500k
2. 'daylight' = temperature of the sun at noon, the colour temperature of the sun ranges from about 2000k at sunrise to about 5500k at it's daily peak, to about 3300k at sunset.

THIS MEANS: Your alexandrite should change colour throughout the day, the best alex show some purple/red flash from the right angles even when the sun is at it's apex.

3.


This is only partly true, although incandescent lights are mostly a thing of the past, most modern LED lights have a 'temperature selection' on them, most commonly: 3000k/4000k/5000k

If they don't have this feature, the kelvin is noted on the box and you can buy warm or cool LED lighting, a 3000k LED changes the colour of a gem just as well as a 3000k incandescent light.

4. 'What lighting to use?': this is the standard, just don't put the light right up to the gem, you need to shine it from about 3 feet away to get the proper colour: https://www.aajewelry.com/51-955.html
Thank you, Skyjems, for very useful information! It appeared, if to measure properly, one of my stones looks grayish purple, because one side stays green, another side changes the color to purple. Sp on the top it looks grayish. It looks beautiful amethyst with candle light, but it's not how we measure it, as I already know. So the right official color is grayish purple! Mystery solved!
 

Jollyroger

Rough_Rock
Joined
Sep 28, 2020
Messages
7
Also, do you guys think, little recut can improve the color change?
Or it is the thing that has to be decided in the beginning by the cutter?
 
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