refractive index for colorless and colored diamond

[qiuqiuqiqi]

I just wonder the refractive index are the same for both colorless and colored diamond?
I saw all the electronic refractive meter in the market are for colorless diamonds only.
I have a violet/red stone for many years. It is very hard.
It can scratch on ruby.
But RI cannot meet 2.41

I also have a black diamond. the RI is 2.43

 

[oncrutchesrightnow]

Cool question. Some expert please answer.

Any pics of the stone? Do you see fire?

 

[qiuqiuqiqi]

 

[qiuqiuqiqi]

Cool question. Some expert please answer.

Any pics of the stone? Do you see fire?

 

[Bron357]

RI for diamond is definately around 2.41, if you can’t get that as a reading then it’s unlikely diamond.
what is the RI?
If it’s around 1.76/ 1.78 it’s likely corundum, probably lab created corundum.
If you have a dichroscope you can also check for double refraction, that’s another indicator of corundum if doubly refractive.
However, you should never test any gems for hardness using other gems as it’s a destructive test to one or the other. In any regard corundum can scratch corundum but testing this way is not recommended.

 

[oldminer]

All diamonds, regardless of body color, have the same RI.

A state of the art thermal probe tool can reliably separate diamonds from CZ, moissanite, and corundum. Be aware that the least costly of these probes will fail to separate moissanite from diamond. Many older models are still being produced and sold because they are so inexpensive. You can get in trouble by not being observant of what each can detect.

Reflectivity meters are not really professional level stone identification devices. They have their uses, but are secondary to more accurate equipment used by professionals. Many professionals own reflectivity devices but more for curiosity and optional testing rather than for definitive identification of gems.

As Bron357 suggested, purchasing and learning to use a dichroscope properly ought to be far more useful and not a costly investment. Just by being able to establish a stone is doubly or singly refractive eliminates many false possible identifications.

 

[qiuqiuqiqi]

thank you all.
Yes I am a destroyer. I like to play with all my collection.
I just brought a yellow diamond on-line with GIA.
It is on the way to me.
So, I will do a little testing and I will let you know which one is harder.
I think this is original idea of Mohs hardness scale.

 

[oldminer]

With a diamond you will find "hardness" varies according to the direction you are attempting to scratch. You can readily damage the surface of a diamond using a diamond test point while testing the hardness of a diamond in an incorrect direction.

The Mohs hardness system is a relatively crude determinant of hardness. I suggest you read about it to fully grasp that it is an approximate system, but hardly a detailed one. Don't begin testing diamonds for hardness unless you are willing to scratch them. You will find nothing in doing it compared to what you can read about without creating any damage.

 

[yssie]

All diamonds, regardless of body color, have the same RI.

A material’s absorption spectrum and its refractive index are actually causally related… Changing the colour of a material through which light can pass will change its refractive index for every wavelength of inbound light. The Kramers-Kronig relations define this relationship explicitly and the effect for almost all materials is most pronounced when the material is absorbing shorter wavelengths (ie. High-frequencies - is coloured violet or blue) - in this case the refractive index of shorter inbound wavelengths is especially raised.
Pedantic, I know. The real-world variance is measurable but small. This just happens to be a topic I'm rather familiar with.

A state of the art thermal probe tool can reliably separate diamonds from CZ, moissanite, and corundum. Be aware that the least costly of these probes will fail to separate moissanite from diamond. Many older models are still being produced and sold because they are so inexpensive. You can get in trouble by not being observant of what each can detect.

Reflectivity meters are not really professional level stone identification devices. They have their uses, but are secondary to more accurate equipment used by professionals. Many professionals own reflectivity devices but more for curiosity and optional testing rather than for definitive identification of gems.

As Bron357 suggested, purchasing and learning to use a dichroscope properly ought to be far more useful and not a costly investment. Just by being able to establish a stone is doubly or singly refractive eliminates many false possible identifications.

+100. Calibrating most of the equipment you'd use to measure reflectance is a #Task unto itself that most hobbyists wouldn't have the materials to do anyway!

 

[yssie]

One thing I can say for sure - this is NOT a diamond.

 

[denverappraiser]

Using one stone to scratch another to see which is harder is almost always a bad idea. There are other variables, as Dave points out above, that render this 'test' nearly useless. More importantly, the end result is damage to one, and possibly both, stones.

 

[John Pollard]

Using one stone to scratch another to see which is harder is almost always a bad idea.

The drivers at monster truck demolition derbies might disagree.
Of course they have mechanics standing by.

 

[Garry H (Cut Nut)]

A material’s absorption spectrum and its refractive index are actually causally related… Changing the colour of a material through which light can pass will change its refractive index for every wavelength of inbound light. The Kramers-Kronig relations define this relationship explicitly and the effect for almost all materials is most pronounced when the material is absorbing shorter wavelengths (ie. High-frequencies - is coloured violet or blue) - in this case the refractive index of shorter inbound wavelengths is especially raised.
Pedantic, I know. The real-world variance is measurable but small. This just happens to be a topic I'm rather familiar with.

+100. Calibrating most of the equipment you'd use to measure reflectance is a #Task unto itself that most hobbyists wouldn't have the materials to do anyway!

Hi Yssie, We measure RI with a specific wavelength of light (The sodium yellow D line that is emitted from the sun and filters are used on refractometers) - so the color really does not have much to do with RI (I think).
Dispersion though is measured by the difference in RI between B red and G violet suns radiation which are 2.407 and 2.451 respectively. The difference is 0.044 and is the dispersion of diamond.
We use the yellow filter on spectroscopes because high dispersion gems have wide fans of color (not that we have diamond measuring refractometers - although they would be possible to make using moissanite.

 

[yssie]

Hi Yssie, We measure RI with a specific wavelength of light (The sodium yellow D line that is emitted from the sun and filters are used on refractometers) - so the color really does not have much to do with RI (I think).
Dispersion though is measured by the difference in RI between B red and G violet suns radiation which are 2.407 and 2.451 respectively. The difference is 0.044 and is the dispersion of diamond.
We use the yellow filter on spectroscopes because high dispersion gems have wide fans of color (not that we have diamond measuring refractometers - although they would be possible to make using moissanite.

I see what you're saying. The "refractive index of diamond", as a concept, is stated to be the amount that diamond material bends a D-line wavelength.

Yes, we're on the same page here - the amount that the material bends the red wavelength is different (less) than the amount that the material bends the blue wavelength - the index of refraction of red light through the material is lower than the index of refraction of blue light through the material - yielding dispersion.

But here's what's interesting, I didn't explain very clearly earlier. Take most plastics, most polymers, types of glass, even liquids like water, and measure how much that material bends the red wavelength and the violet wavelength - and let's call those values RI(cr) and RI(cv) for refractive index (clear plastic, red wavelength) and refractive index (clear plastic, violet wavelength). Now if we take that same glass of liquid and dump some inert blue dye in, or we make a new batch of the exact same plastic/glass except it's blue coloured... If we measure how much that new blue material bends the red wavelength and the violet wavelength - call those values RI(br) and RI(bv) for refractive index (blue plastic, red wavelength) and refractive index (blue plastic, violet wavelength), we'll see that for lots of materials RI(cr) is different from RI(br), and RI(cv) is different from RI(bv). Specifically, in almost all cases, RI(bv) is higher than RI(cv) - Kramers-Kronig equalities demand more effect on higher frequencies. All that to say that for different material colours (ie. As the material's own absorption spectrum changes), that material actually bends different incident wavelengths differently! I don't know for sure, but based on the ubiquitousness of this phenomenon I would guess that the amount that an incident D-line wavelength is bent varies with diamond colour (hue and saturation of that colour) as well.

Of course stochastic models for the fourier transform that kk resolves are going to start to fail at real world boundary conditions - validity is considered mathematically universal when absorption is between {zero} and {somewhere less than infinite} - and of no use when the material is actually fully opaque. I don't know if any sort of diamond other than a black diamond would fall into this category though. I guess also diamonds with significant clouds/wisps/internal graining.

I lost you at the moissanite refractometer - can you explain that more @Garry H (Cut Nut)?

Edit - fixing tags and tagging @Serg and @diagem as well.

 

[Garry H (Cut Nut)]

I just wonder the refractive index are the same for both colorless and colored diamond?
I saw all the electronic refractive meter in the market are for colorless diamonds only.
I have a violet/red stone for many years. It is very hard.
It can scratch on ruby.
But RI cannot meet 2.41

I also have a black diamond. the RI is 2.43

Sorry we took over your thread. You stone is 99% synthetic corundum (ruby / purple sapphire) or spinel based on a few factors visible in your excellent photos.

 

[Garry H (Cut Nut)]

I see what you're saying. The "refractive index of diamond", as a concept, is stated to be the amount that diamond material bends a D-line wavelength.

Yes, we're on the same page here - the amount that the material bends the red wavelength is different (less) than the amount that the material bends the blue wavelength - the index of refraction of red light through the material is lower than the index of refraction of blue light through the material - yielding dispersion.

But here's what's interesting, I didn't explain very clearly earlier. Take most plastics, most polymers, types of glass, even liquids like water, and measure how much that material bends the red wavelength and the violet wavelength - and let's call those values RI(cr) and RI(cv) for refractive index (clear plastic, red wavelength) and refractive index (clear plastic, violet wavelength). Now if we take that same glass of liquid and dump some inert blue dye in, or we make a new batch of the exact same plastic/glass except it's blue coloured... If we measure how much that new blue material bends the red wavelength and the violet wavelength - call those values RI(br) and RI(bv) for refractive index (blue plastic, red wavelength) and refractive index (blue plastic, violet wavelength), we'll see that for lots of materials RI(cr) is different from RI(br), and RI(cv) is different from RI(bv). Specifically, in almost all cases, RI(bv) is higher than RI(cv) - Kramers-Kronig equalities demand more effect on higher frequencies. All that to say that for different material colours (ie. As the material's own absorption spectrum changes), that material actually bends different incident wavelengths differently! I don't know for sure, but based on the ubiquitousness of this phenomenon I would guess that the amount that an incident D-line wavelength is bent varies with diamond colour (hue and saturation of that colour) as well.

Of course stochastic models for the fourier transform that kk resolves are going to start to fail at real world boundary conditions - validity is considered mathematically universal when absorption is between {zero} and {somewhere less than infinite} - and of no use when the material is actually fully opaque. I don't know if any sort of diamond other than a black diamond would fall into this category though. I guess also diamonds with significant clouds/wisps/internal graining.

I lost you at the moissanite refractometer - can you explain that more @Garry H (Cut Nut)?

Edit - fixing tags and tagging @Serg and @diagem as well.

Yssie, I copied your comments to a friend who is knowledgeable in such matters:
the author is suggesting that the dispersion (ie the difference in RI from one end of the visible optical spectrum to the other, arbitrarily chosen as the "B" & "D" Fraunhofer lines, of solar hydrogen if I remember correctly?...) is a function of the absorption spectrum, which is just completely incorrect if the composition of the host material has not been substantially altered when its colour was altered, (If the composition was changed when the the colour was changed, then of course, we're not measuring then the properties of the 'same' substance!)

Dispersion of any material is an invariant property characteristic of that substance of that particular composition (may be anisotropic, ie directionally dependent tho), & its (crystallographic) microstructure. (That last rider is included since there are substances with different allotropes or isomorphs, ie same chemical composition but different microstructures & crystllographic & physical properties, such as diamond & graphite or the eight or more different varieties of sulphur or in terms of minerals for example, say, kyanite, sillimanite & andalusite, all chemically identical but with different optical & physical & microstructural properties, & there are many other such examples also of course, for instance the silicas!).

 

[yssie]

The relationship between a material’s (changing) absorption spectrum and its (changing) refractive index is a very well-documented mathematical proof - irrespective of chemical composition alteration. This question is not a matter of what Yssie or @Garry H (Cut Nut)’s Friend might wish to think! Kramers-Kronig is a theoretical equality - here it is in full:

https://www.researchgate.net/publication/241551073_Kramers-Kronig_Relations_in_Optical_Materials_Research

It is a long book. I invite your friend to inspect the matlab samples at the end, especially, if it is possible for him or her to do that, as they outline and solve for exactly these sorts of different scenarios (that is - the noncomplex refractive index, no one bothers with the equation for the imaginary cooefficient because it’s got little real world utility).

However, this mathematical conversation ^ is where my background ends, because I am not a chemist or a biologist! So how alterations to crystal lattice or impurities impact how we perceive colour, those are questions I’m not qualified to opine on.

But then that is another issue isn’t ir? My examples of black diamonds and cloudy diamonds earlier were poor, because those colour changes occur because of material composition changes… Which must impact optics, perceptibly or not by humans. But in the same vein any impurities or inclusions that create colour in diamonds must have chemical or structural causes that throw wrenches into a theoretical proof.

So whether we solve for RI with changing colour sans any composition change (but per your friend’s word that this is real-world impossible) or we assume physical changes that create colour… We come to shifting RI as diamond colour changes no matter which way we go!

 

[Garry H (Cut Nut)]

You mean thisin a simpler form? https://www.rp-photonics.com/kramers_kronig_relations.html
When we are discussing diamond:
I am pretty sure that in abnormal conditions RI can change - such as under huge loads. I suppose intense internal stress is causing some tiny changes in RI - but above my pay grade.
The only inbound wavelength that is relevant is sodium yellow D. I do not believe the color can change the RI.
RI by definition is measured at that wavelength / frequency.

 

[Garry H (Cut Nut)]

I think we are at crossed purposes Yssie - in gemstones we use this single wavelenght un waveringly:
The Fraunhoffer D line is 589.29 nm

Fraunhofer lines - Wikipedia

en.wikipedia.org

Other materials than gem stones apparently use different standards - for example my friend sent a Schott Glass doc and it mentions:
"The refractive index is a function of the wavelength. The most common characteristic quantity for characterization of an optical glass is the refractive index n in the middle range of the visible spectrum. This principal refractive index is usually denoted as nd – the refractive index at the wavelength 587.56 nm or in many cases as ne at the wavelength 546.07 nm."

​

 

[yssie]

The only inbound wavelength that is relevant is sodium yellow D. I do not believe the color can change the RI.

RI by definition is measured at that wavelength / frequency.

Okay! I agree we may be talking across each other here.

If we say that white light - the whole spectrum of wavelengths - is going into the diamond, then as long as the diamond stays mostly transparent, the amount that that diamond bends each wavelength that went in may change as the diamond’s colour changes, even if the colour change magically isn’t accompanied by any chemical or structural change. This is what that proof is saying (and I think also the page you linked that simplifies the real equation, but I haven’t looked properly). I say “may change” to be careful, because some wavelengths are bent more and some are bent less and some are bent exactly the same amount as material colour changes.

But if we hold the light going into the diamond static at 589nm, which is the *definition* of RI for diamond, only this one wavelength, then as long as the diamond stays mostly transparent, if you say nothing changes as diamond colour changes, then I will believe that! That would mean that this is one wavelength that is bent exactly the same amount even as material colour changes.

Garry, I appreciate the time you have taken to explain this more! I’m sure I am not the only reader who did not understand the implication of using this specific definition.

 

[Starstruck8]

Okay! I agree we may be talking across each other here.

If we say that white light - the whole spectrum of wavelengths - is going into the diamond, then as long as the diamond stays mostly transparent, the amount that that diamond bends each wavelength that went in may change as the diamond’s colour changes, even if the colour change magically isn’t accompanied by any chemical or structural change. This is what that proof is saying (and I think also the page you linked that simplifies the real equation, but I haven’t looked properly). I say “may change” to be careful, because some wavelengths are bent more and some are bent less and some are bent exactly the same amount as material colour changes.

But if we hold the light going into the diamond static at 589nm, which is the *definition* of RI for diamond, only this one wavelength, then as long as the diamond stays mostly transparent, if you say nothing changes as diamond colour changes, then I will believe that! That would mean that this is one wavelength that is bent exactly the same amount even as material colour changes.

Garry, I appreciate the time you have taken to explain this more! I’m sure I am not the only reader who did not understand the implication of using this specific definition.

This issue puzzled me for a while. KK must be true. (It’s a theorem, right?) But for diamonds of different colours, r.i. measured at the standard wavelength doesn’t change noticeably with the colour. Same for sapphires, or for garnets with the same basic composition, etc. So what is going on?

Answer: scale. The imaginary part of the refractive index relates to absorption over a path length of a wavelength. [Strictly, the formula is: absorption coefficient = (imaginary part of r.i.)/(4*pi*(vacuum wavelength)).] The wavelengths of visible light are of order of magnitude 0.1 – 1 micron. For typical coloured diamonds and other gemstones, the path lengths for noticeable absorption of visible light are of order mm to cm. So the complex part of the r.i. is tiny [less than 10^(-3)]. So the effect on the real part as calculated from KK will also be tiny.

Quick summary: there is a genuine effect, but it’s usually negligible in practice. Note also, there is nothing special (from this point of view) about 589nm – it’s just conventional and convenient.