Does colour influence the refractive index of a diamond?

[tanalasta]

Technical question and a re-write of a philosophical post I have had to edit due to forum policies.

1. How does diamond colour influence the refractive index of the diamond (assumed to be 2.417 or 2.418 depending on which way you round). e.g. D vs F or D vs K and white vs say yellow or pink.

2. Has anybody studied the 'cut' adjustments in relation to the crown and pav relationships compared to the Tolkowsky ideal of 14.5 / 40.75 degrees that are required if on was cutting a non-D coloured diamond of a fancy colour.

Diamonds are cut according to many factors including maximising profit from a rough's shape, inclusions, strain etc... and that there are many different proportions that have been studied and may still be lovely AGS0 type diamonds. The differences are also likely to be minute and insignificant So this is a purely philosophical question. It's fascinating stuff - perhaps I should give up my day-job, study gemmology, move to Melbourne and ask Cut-Nut for a job *wonders how much it pays*

Would I be right in assuming that the lower the refractive index, the shallower the combination should be and vice versa?

 

[belle]

the refractive index does not change with the color grade of the diamond, it is constant.

 

[rjdodd]

At the risk of going out on a limb...

Refractive index is itself a function of frequency, given that different color diamonds pass different frequencies in different amounts, the average refractive index is goind to vary with the color of the diamond.

The question is whether it''s detectable and at what level. In general I''d guess you''re going to have *very* small variations between say an intense red diamond and a blue one (detectable in a lab maybe). Anyone got a couple they could send me so I can check out my theory?

 

[CaptAubrey]

As a matter of fact, RI does change with color.

Rjdodd is correct. According to a mineralogy refererence manual I have, red light (643 nm) through diamond has an RI of 2.411; deep blue/violet light (441 nm) has an RI of 2.447. It''s this difference in RI that causes dispersion, because light of different colors is refracted at slightly different angles, separating a ray of light into its component colors.

This is a difference that is easily measurable with the right kind of refractometer, but it''s not a cut issue.

 

[oldminer]

Good info here. This is the sort of dialogue that extends the inner workings of diamonds to everyone who is interested enough to make the effort.

Diamonds are cut differently to enhance visible color in fancy colors compared to the way they are cut in near colorless diamonds.

 

[pricescope]

Capt, if you model a green light going thru D-colored and H-colored diamonds, permittivity (which is actually what RI is a function of) would be the same, well it''s a guesses of a housewife who has no access to scientific data published on the net these days, all the mags are by a subscriptions now.

 

[rjdodd]

What I was trying to say was that the RI of a given colour of light does not change, but the RI is diferent for different colours. As a result the average (over all colours - or all transmitted light) RI is going to change depending on the transmission spectra of the stone.

A simplified example: for a blue stone the RI is 2.447 since is ''only'' transmits blue light, and for a red stone the RI is 2.411 for the same reason.

 

[kenny]

I read here recently that dirt changes the RI.
A diamond with no leakage when clean may leak some light when it is dirty since the dirt actually changes the angle that the light bounces at.

Since 99.9% of diamonds spend 99.9% of their lives dirty maybe they should cut them at angles to compensate for dirt.

 

[strmrdr]

Date: 1/23/2007 4:40:58 PM
Author: kenny
I read here recently that dirt changes the RI.
A diamond with no leakage when clean may leak some light when it is dirty since the dirt changes the angle that the light bounces at.

Since 99.9% of diamonds spend 99.9% of their lives dirty maybe they should cut them at angles to compensate for dirt.

Now you sound like Garry LOL

 

[strmrdr]

In other discussions its not too big an issue until you hit or get near the fancy colored range.

The tightest variation the pavilion of diamonds is cut too(~.10 to .30 degrees on the best of the best) is likely a bigger difference than the difference RI would make D to J.

 

[rjdodd]

I should really leave this to the real experts

... But because I''m an arrogant knowitall I won''t.

Dirt doesn''t change the RI, but dirt also doesn''t just reduce brilliance by blocking light from reaching the diamond. The brilliance is a function of total internal reflections within the diamond, and the angles required to TIR are related to ratios of RI for the interface (on clean interface that would be diamond compared to air).

When there is dirt on the surface of the diamond (finger grease for example) the ratio is different and the cut angles are no longer appropriate for TIR - net result is less brilliance. More of problem for dirt on the bottom of the stone than the top.

So i think dirt / grease / etc. will effect brilliance (and I use the term brilliance as a non expert - brightness) in more than one way. Dirt

is the enemy on many fronts.

 

[strmrdr]

yep dirt on the bottom makes a bigger difference as long as the crown isnt blocked to the point of not getting light. Also people tend to clean the crown more.

Garry Holloway has a theory as yet unproven beyond a reasonable level that shallow diamonds handle dirty pavilions better.

 

[CaptAubrey]

Date: 1/23/2007 3:41:53 PM
Author: Pricescope
Capt, if you model a green light going thru D-colored and H-colored diamonds, permittivity (which is actually what RI is a function of) would be the same, well it''s a guesses of a housewife who has no access to scientific data published on the net these days, all the mags are by a subscriptions now.

No, I don''t think that green light is going to be affected by H color. A typical H-color diamond gets its color from an absorption in the blue region of the spectrum; if you''ve got, say, green light at 600 nm, it won''t be affected by that absorption, which is usually at 415 nm.

I''m not sure the same would be true of, say, blue light. But I''m not the physicist here.

 

[rjdodd]

Date: 1/23/2007 5:00:40 PM
Author: strmrdr
yep dirt on the bottom makes a bigger difference as long as the crown isnt blocked to the point of not getting light. Also people tend to clean the crown more.

Garry Holloway has a theory as yet unproven beyond a reasonable level that shallow diamonds handle dirty pavilions better.

I''d tend to agree with Garry - but i know nothing about diamonds (or cutting) so that doesn''t mean much

. I''m cmoing at this from a purely optics view point. From a couple of scribbles on a piece of paper (not exactly scientific or precise) shallow cuts are likely to have incident beams from above strike the pavillion surface at shallower angles - which would make TIR more likely depending on the type of dirt. Steep diamonds may have the first reflection TIR ok but the second (doodles at work here) at too high an angle for TIR so we get leakage. Since a dirty surface requires a shallower incident angle to get TIR. So for two diamonds that have the same optical response (one steep & one deep - doubt this is even possible) the steeper one will lose more TIR from the dirt.

Take everything I say (and not just about this) with a giant pinch of salt.

With regards to tanalasta and the original questions, and your comments - all this is likely to be a rounding error compared to the quality of the cut.

 

[pricescope]

Coming back to the original question, a real crystal structure, optic wise, is complicated enough (even a theoretical structure as "simple" as diamond) to additionally throw dirt

into equation.
For example, D-color I-clarity diamond not necessarily has higher RI than let's say J-color IF-clarity, there are structural issues beyond coloring centres that can influence material's permittivity , especially for not given direction or wavelength of light, temperature or internal strains and other structural components...
Again, have no data, just guessing.

 

[adamasgem]

Date: 1/23/2007 10:34:32 AM
Author:tanalasta
Technical question and a re-write of a philosophical post I have had to edit due to forum policies.

1. How does diamond colour influence the refractive index of the diamond (assumed to be 2.417 or 2.418 depending on which way you round). e.g. D vs F or D vs K and white vs say yellow or pink.

2. Has anybody studied the ''cut'' adjustments in relation to the crown and pav relationships compared to the Tolkowsky ideal of 14.5 / 40.75 degrees that are required if on was cutting a non-D coloured diamond of a fancy colour.

Diamonds are cut according to many factors including maximising profit from a rough''s shape, inclusions, strain etc... and that there are many different proportions that have been studied and may still be lovely AGS0 type diamonds. The differences are also likely to be minute and insignificant So this is a purely philosophical question. It''s fascinating stuff - perhaps I should give up my day-job, study gemmology, move to Melbourne and ask Cut-Nut for a job *wonders how much it pays*

Would I be right in assuming that the lower the refractive index, the shallower the combination should be and vice versa?

To try to address your original questions

1) It is the other ay around, the Refractive index can influence the percieved color..
The RI varies with wavelength, and the total pathlength inside the diamond varies with R3efractive Index, and the total internal absorption at a particualar wavelength varies nonlilinearlywith pathlength, resulting in perceived color.

2) Yes, people have, and cutters have learned how to manipulate facet angles to concentrate or increase color in fancies, lucky Vivids they are called. Theoretically, a D color has a different optimal facet arragement than a G color. but the present complexities of raytracing, as well as time and measurement accuracy preclude any serious look see, as other factors, light what lighting envirionment you optimize to, come into play.. Would be a neat job for NSA computers.
In the meantime you can play with these factors with DiamondCalc and/or write your own software, as I did both.

3) The differences in internal absorption due to cutting are not necessarily minute. For example, most diamonds today have faceted girdles because the faceted giirdles allow for total internal light reflection (and not light loss), and can actually result in a better color grade, all other factors being equal..

Different Gems are cut to different sets of nominal facet angles, for the same shape, because of the difference in Refractive indicies, among other things