Facet Yaw

[JohnQuixote]

It may be helpful to illustrate an effect that can influence the physical symmetry of any round brilliant, and acute precision in Hearts & Arrows cuts.

Brian Gavin terms the effect Facet Yaw.

During polishing, the diamond is held on the dop, which is in turn held by the tool known as the tongue. A cutter can use a screw on the tongue to “cheat” the facet (proper cutter’s use of that term)

for several reasons: Either because the facet is not "running" during polishing because of graining (meeting resistance against the grain) – or the cutter wants to make improperly-cut facets appear to fit properly – or in an attempt to help retain weight.

This "cheating" involves rotating the index wheel between the teeth so the facet may be polished from side to side (improper) rather than from culet to girdle or girdle to culet (proper). This causes an azimuth shift. The face will not be flat, because the starting point (edge) is ground (polished) away “up” until the facet reaches the other side. The facet begins to yaw: One end is deeper than the other, therefore the facet’s face is not flat anymore. It has yaw (illustrations below).

Practical Examples: A “tilted table” type effect (linked here) or parts of these “Phony” hearts & arrows effects (linked here).


Important safety tip!…The only method of directly observing facet yaw is with (drum roll please)…a Hearts & Arrows viewer. Sarin, Ogi, etc. can pick up yaw, but it is difficult to interpret without experience and understanding of the numbers.

So… This is why some of us preach about patterning being so important (Brian’s catch phrase is “It’s all in the hearts”) and why the H&A tool is so useful. That poor little H&A viewer has been referred to as a “toy” by some who don’t understand everything it reveals, but there is no other device that tells one so many things about a diamond’s construction…

1. Precision of Hearts & Arrows patterning (if present).
2. Physical Symmetry / Proper averaging of opposite slope angles.
3. Facet Yaw / Bad patterning.
4. Lower Girdle Facet lengths (which directly influence the balance of brilliance and fire each diamond will have).

You get all of this information in one pretty picture. No other device tells this info so immediately and so clearly. You’ll need to understand how to recognize the elements you are seeing, but anyone can have an instant perception of how good the craftsmanship of a stone is without too much training. The H&A viewer is the “Idealscope” of symmetry and craftsmanship.

This often goes unmentioned: You can use a H&A viewer to determine the symmetry, yaw and lower girdle facet lengths of any diamond, even if it does not have Hearts & Arrows. All stones have some kind of patterning. We can tell from that patterning what is happening in the stone with regard to craftsmanship.


Another item relating to the main subject of this post: “Hot spots” talked about in Brilliance Scope reports can be correlated to facet yaw. Yaw sends tiny needles of light in unpredictable places. The machine “sees” a needle-like little flash and, misinterprets it. This is another reason (aside from relevance) that I consider reports by devices of this ilk to be spurious, whereas H&A viewer and IdealScope images are reliable.

Just as Garry Holloway’s IdealScope has helped improve standards across the board (including ours), my hope is that there will be increased understanding of the H&A viewer so more people will have the ability to immediately recognize proper patterning and proportions without distortion in a diamond. Such awareness on a broad scale may help put demand on the suppliers to maintain high standards – a crusade for many of us in the PriceScope community.

 

[JohnQuixote]

Facet Yaw Illustration

 

[JohnQuixote]

Facet Yaw Example - Note: The yaw in H&A view #2 is *extreme*

 

[JohnQuixote]

IdealScope, Clarity and H&A views of a diamond with "True" H&A, no facet yaw and lower girdle facet 80%.

 

[JohnQuixote]

.

 

[JohnQuixote]

Hearts (pavilion) view displaying moderate yaw and lower girdle facet % >80%

 

[JohnQuixote]

Arrows (crown) view displaying yaw.
[Hearts (pavilion) view is typically more telling]

 

[Garry H (Cut Nut)]

Sir John that is a very complex descprition that would confuse any one who even understands what you are discussing.
Dont worry folks - he will edit it later

Watch this space

 

[strmrdr]

I found it easy to understand but have a couple comments and questions.
Some of which I know the answers too but others might not.
comments:
The yawed diamond if you judge it by the ideal-scope is still a very well performing diamond certainly better than average.
Just dont pay a h&a premium price for it.

Questions:
If im currect in reading that yaw causes light to break up and be returned in random directions could this not produce more fire at the expence of brightness?

Which facets are usualy yawed?
What are the effects of the less commonly yawed facets being yawed?

What pattern to look for on the sarin numbers if an h&a image isnt available?

Will a diamond with yaw still get an id/ex in symetry from the labs?

What does yaw look like under the microscope? Is a picture possible?

Wouldnt this make it even more critical to get the real h&a images from your vendor and not go by stock images? (remember that not all vendors are equaly trustworthy) :}

 

[valeria101]

Looked once, twice... and I still don''t see what the red arrows are pointing at on the Ideal Scope. It seems that the longer upper girdle facets are responsible of the deeper cleft on the harts. But what does "yaw" do ?

It would help to have the same pointed at on the larger H&A pictures.

I am trying to immagine that it is the more "random" distribution of red and white reflections around the harts and arrows that one should be looking for, but is this so?

 

[noobie]

Date: 11/26/2004 8:41:49 AM
Author: strmrdr


The yawed diamond if you judge it by the ideal-scope is still a very well performing diamond certainly better than average.

Just dont pay a h&a premium price for it.


Questions:

If im currect in reading that yaw causes light to break up and be returned in random directions could this not produce more fire at the expence of brightness?


Which facets are usualy yawed?

What are the effects of the less commonly yawed facets being yawed?


What pattern to look for on the sarin numbers if an h&a image isnt available?


Will a diamond with yaw still get an id/ex in symetry from the labs?


What does yaw look like under the microscope? Is a picture possible?


Wouldnt this make it even more critical to get the real h&a images from your vendor and not go by stock images? (remember that not all vendors are equaly trustworthy) :}

Yes, I would tend to share this view. Interested in the answers as well.

 

[JohnQuixote]

Date: 11/26/2004 8:43

0 AM
Author: valeria101
Looked once, twice... and I still don''t see what the red arrows are pointing at on the Ideal Scope. It seems that the longer upper girdle facets are responsible of the deeper cleft on the harts. But what does ''yaw'' do ?

It would help to have the same pointed at on the larger H&A pictures.

I am trying to immagine that it is the more ''random'' distribution of red and white reflections around the harts and arrows that one should be looking for, but is this so?

Ana,

You are seeing the lower girdle facet issues correctly.
I will provide some larger, examples of yaw.

FYI: Yaw shows up as distortions in the pattern: "Twist," in the pattern (meet points appear not to meet because the reflection is "twisted"), reflections in the pattern where there should be none and reflections which are uniform in good patterning appearing as not uniform.

 

[JohnQuixote]

Close up showing a pavilion facet with yaw.

1. The bottom half (outlined in green) of the pattern has "twist" (meet points appear not to meet because the reflection is "twisted").
2. On the left, reflections which are uniform in a good pattern appear as not uniform.

(disregard the 2 arrows on the right for now)

 

[JohnQuixote]

To further illustrate, we take the area outlined in green and move it left to the meet point. We can see the amount of yaw. The profile of the yawed reflection is much smaller, even though the surface area should be the same with good patterning.

 

[JohnQuixote]

This shows to what degree the angle of the yaw is occuring.

 

[JohnQuixote]

Unzooming a bit to show a pavilion facet with similar yaw.

A: Yaw is evident as distortion in the pattern. Arrows below "A"s indicate where meet points are distorted.
A1: End of the yawed facet.
A2: Yaw is evident in the reflection of 1A appearing where no reflection should appear.
B: Yaw is evident as reflections which are not uniform.

This is an AGS0 stone. Graded "Ideal" in both Polish and in Symmetry.

That's why I believe this definition of symmetry is important to impart.

 

[JohnQuixote]

Good patterning. No yaw.

 

[JohnQuixote]

Date: 11/26/2004 8:41:49 AM
Author: strmrdr


I found it easy to understand but have a couple comments and questions.
Some of which I know the answers too but others might not.

Sir Strm, ok. You in italics, me in standard:

The yawed diamond if you judge it by the ideal-scope is still a very well performing diamond certainly better than average. Just dont pay a h&a premium price for it.

100% correct (that's why there is such a thing as "expert selection")

We’re not talking about light return - we have IdealScope for that. We are talking about qualities of visual balance within light return, including (but not limited to) contrast and levels of observable brilliance/fire.

If im currect in reading that yaw causes light to break up and be returned in random directions could this not produce more fire at the expence of brightness?

Maybe in some lighting conditions, but since diamonds are not viewed in static conditions this is going to vary. What we do know is that yaw occurs at the expense of contrast, which alters the visual balance of the stone. On a cloudy day there will not be additional fire, and the contrast will suffer as well. For that matter, depending on location or severity of the yaw, that returned light may not even reach the viewer's eye.

Which facets are usualy yawed?

Any facet can be yawed, but it happens primarily in main pavilion and lower girdle facets. For an extreme example, see my “Facet Yaw, Practical Example” second diagram above where just about every facet is yawed.

What are the effects of the less commonly yawed facets being yawed?

I'm not sure what you mean here, Strm. Can you be more specific?

What pattern to look for on the sarin numbers if an h&a image isnt available?

You have to have a much greater degree of detail that may be available to the person running the Sarin but is not available to or interpretable by most.

Will a diamond with yaw still get an id/ex in symetry from the labs?

YES! This is one of the most important things to understand. The cutter has "cheated" and yawed the stone to get the lab's symmetry grade, but he has sacrificed physical symmetry. The outlined examples above are an AGS 0.

What does yaw look like under the microscope? Is a picture possible?

What does light leakage look like under the microscope?

It's not easily seen. That's why the H&A viewer is used.

Wouldnt this make it even more critical to get the real h&a images from your vendor and not go by stock images? (remember that not all vendors are equaly trustworthy) :}

Long answer: As casual consumers become more educated with regard to yaw and H&A images it will become increasingly important. As it stands, not many people know how to interpret these things we’re discussing. But then, a few years ago not everyone knew how to interpret IdealScope images.

The short answer, Strm, is yes.
(which means you can eventually say "I told'ja so.")

 

[Richard Sherwood]

What an excellent teaching.

Thanks John.

 

[Patty]

Very interesting stuff.

 

[strmrdr]

Thanks for the answers John.

What are the effects of the less commonly yawed facets being yawed?

I''m not sure what you mean here, Strm. Can you be more specific?

Lets say the star facets are yawed how would this show up or even would it?

 

[Garry H (Cut Nut)]

Date: 11/26/2004 3:47:43 AM
Author:JohnQuixote
The only method of directly observing facet yaw is with (drum roll please)…a Hearts & Arrows viewer. Sarin, Ogi, etc. can pick up yaw, but it is difficult to interpret without experience and understanding of the numbers.

John you can see it in the girdle wave too with a 10x loupe

John you can see it in the girdle wave too with a 10x loupe.

I think it is easier to say that yaw (indexing) is when the azimuth of a facet is changed (with or without changing the facet slope).

Indexing is normally set at 360 divided by 8 for main facets (every 45 degrees), and at the in between positions for the stars (statring at 22.5 degrees and then every 45 degrees).

The upper and lower girdle facets are normally indexed at 11.25 degree and then every 22.50 degrees (there are 16 on a RB).
It is the UG''s in particular that are more commonly indexed. However a polisher may need to tilt say one kite facet 1 degrees to say the right to remove an inclusion. This means that facet will not meet properly with its neighbouring stars and UG''s - so the polisher indexes the adjacent right side facets by 0.8 degrees, then the next set along by 0.6 degrees and the next by 0.4 and the next by 0.2 until we have no easily seen problem with 10x analysis. The same thing can be done to make the pavilion and LG"s align so there is no twist deviation.

A normal Sarin or Ogi scan will not detect this yaw because it does not report the azimuth variations. And the facet sllope or crown and pavilion angles may not have changed.

 

[Garry H (Cut Nut)]

This is an actual example of a ''swindled stone''. . The crown was completely re indexed like in the example above. And the pavilion angle had 0.2 degrees removed (I would have liked it to have ben .04 degrees - but we were worried about falling under 2.000ct) and had been aligned better than in the original case. the grades are both from GIA.

The customer had bought the stone as D IF with an Israeli cert. He was upset when we told him we doubted it - he asked us to source him a nice 2ct D IF, which we did, and we took the trade in for recutting.
He was so impressed he bought it back from us at the trade up price, plus our costs for recutting and regrading etc.

It always needs to be stated - there is more skill in swindling like this, than in simply cutting H&A''s on an auto dialit or process line. i am constantly in awe of the skill these people exhibit. (and btw they are often teams of several people doing each part of a process).

 

[valeria101]

Great follow up !

The fact that optical symmetry is not the same with what lab reports call "symmetry grade" has been tossed around for quite a while with no explanation.

As far as I undersatnd, this thread begins the story. Is there much more in store?

How about this Q:

The description of facet yaw shows that the certified symmetry grade is not suficient to obtain optical symmetry. Clear enough. But is it one necessary to the other: can there be diamonds with great H&A and "good" symmetry grades on lab reports ? A recent therad on crown and pavilion angle variation makes me guess this could be the case - hence the Q

 

[Garry H (Cut Nut)]

Yaw seems to be a silly word.
Our definition of Yaw should be "See indexing" or "See Girdle cheating" or "See painting, gouging" etc etc
Yaw = indexing = changing azimuth from the usual position of 0, 11.25, 22.5, 33.75 or 45 degrees (etc) - This means the dierection each facet FACES.

Yaw does not mean the facet is not flat.
Yaw does not mean the facet has been polished up, down, left or to the right.

Ana I am very sorry - after reading your question I relaized have not made myself clear:
can there be diamonds with great H&A and "good" symmetry grades on lab reports ?
I have many times linked people here http://www.gemology.ru/cut/english/symmetry/6.htm

But I forgot that during the editing yuri decided not to be offensive to any particular lab (in this case GIA) by stating that this actual stone was awarded GIA good for symmetry. I have copied the text relevant here - the tables and images are at the link.

"The diamond No.2 is owned by one of the authors (G.H). The parameters of this diamond with tilted table are shown in the Table 4. The crown and pavilion axes are tilted from table axis at the different angles. As the result the significant (approximately 1 degree) difference will be obtained if the angles will be measured in the base of the table plane. Meanwhile a good quality of such a stone is confirmed by «hearts and arrows» patterns obtained with structural illumination after its position was corrected on order to match the viewer’s axis (Fig. 3)

Table removed

Table 4. Rows 1 and 2 – angles measured by a Sarin scanner; row 3 – the sums of the angles of opposite facets; row 4 – the average values of these angles.

Fig. 3. a) Snapshot of the diamond No.2, made by means of an Idealscope. The diamond is positioned so that its table is perpendicular to the optical axis of the camera. An off-axis displacement of the culet and pale regions of partial light leakage through the pavilion can bee seen. b) Snapshot of the same diamond slightly rotated in order to compensate for the table tilt. The degree of optical symmetry became much higher, as a result. c) and d) Snapshots of the diamond made by means of a “Hearts and Arrows viewer” device, which demonstrate perfect «hearts and arrows» patterns."

Doe that make your H&A''s question clearer Ana?
(that example has nothing to do with yaw btw - but in fact it would be possible to say that in comparison to an average axis, or say a the all important pavilion angle averaged axis - that this stone has a table yaw of about 1/2 a degree)

 

[mdx]

For those following this thread, here is a simplified chart of the normal polishing directions for 4point 3point and 2 point crystal structures.
The number of points is essentially the shape of the table after initial sawing or cleaving prior to bruting (rounding)

An impotant point Gary mentioned is no matter how you use the "cheater" to modify the facet, the facet is always flat irrespective of angular or azimuth change.

Johan

 

[JohnQuixote]

Hi Johan (say, what a great name)

Nice to have 'another from down under' in the thread.

Flat...Let's have a rest of how this term is being used.

A yawed facet may be flat unto itself. But a yawed facet is NOT flat in relation to the line on which it SHOULD have been run (see diagrams at the beginning of the thread). From the starting point to the end point there will be a difference in the elevation of the facet. Relative to the axis of how it should have been cut, and relative to adjacent facets, it is NOT flat (there are micron differences in the flatness) it is yawing in one direction or another. This is always reflected in the patterning.


Thanks for the graining diagram. It might be a bit confusing for anyone who does not know what the arrows represent, however...

The arrows on that diagram show the direction the wheel must go when touching the facet, however the first contact point (for pavilion facets) will either be at the girdle or at the culet, and proper polishing will happen top to bottom or vice-versa.


Okay Johan, here's a question which will be helpful to our collective discussion: On a 2-point stone can you explain - for the layman - how to get the table to run?

 

[JohnQuixote]

Date: 11/28/2004 12

3

7 AM
Author: JohnQuixote

Relative to the axis of how it should have been cut, and relative to adjacent facets, it is NOT flat (there are micron differences in the flatness) it is yawing in one direction or another.

(correlation to "axis" and "yaw")

 

[JohnQuixote]

Date: 11/26/2004 8:43:16 PM
Author: Richard Sherwood
What an excellent teaching.


Thanks John.

Rich - Thank you, sir.

The best way to learn is to teach.
This is fascinating stuff to me.

 

[JohnQuixote]

Date: 11/26/2004 9:24:26 PM
Author: Garry H (Cut Nut)

John you can see it in the girdle wave too with a 10x loupe.

Maybe, but why would you want to Garry? Remember it takes 5 different facets working in harmony to create one pattern in the pavilion. A loupe won''t show how a yawed facet interacts with others (which is the point of patterning). You won''t see resultant stray reflection either, or be able to gauge the level of non-uniformity in the symmetry. All the above are seen in the H&A viewer without going through so much effort or training. Brian has shown me light leakage through a loupe, but the point here is to make it easier for consumers to see these things - that’s what this whole forum is about, right?

Thinking about that reminded me of another point about viewing pavilion patterning: One of the most difficult things about analyzing a RB is getting the stone to be level. When taking clarity photos, arrows images or IdealScope images it takes effort to make certain it is completely level in the table-up position (or you get skewed results).

But...Viewing the hearts patterning is done table-down! The hearts image is the only view that allows user-friendly, accurate assessment of the way the facets behave in relation to each other. You are looking at complete facet interaction in the hearts imagery. It''s where you see yaw, harmony, and consistency of shape. In a stone with good patterning you can even see effects of a yawed (you say tilted) table in the viewer without a Sarin machine, because you will have to skew the angle of your head somewhat to try and bring the pattern into a harmonious position (which may or may not be possible).

I think it is easier to say that yaw (indexing) is when the azimuth of a facet is changed (with or without changing the facet slope).

Umm, Sir Garry, in the initial post I said >

I think we''re saying the same thing. Maybe you skimmed it. I know you''re a busy guy

Indexing is normally set at 360 divided by 8 for main facets (every 45 degrees), and at the in between positions for the stars (statring at 22.5 degrees and then every 45 degrees).

The upper and lower girdle facets are normally indexed at 11.25 degree and then every 22.50 degrees (there are 16 on a RB).
It is the UG''s in particular that are more commonly indexed. However a polisher may need to tilt say one kite facet 1 degrees to say the right to remove an inclusion. This means that facet will not meet properly with its neighbouring stars and UG''s - so the polisher indexes the adjacent right side facets by 0.8 degrees, then the next set along by 0.6 degrees and the next by 0.4 and the next by 0.2 until we have no easily seen problem with 10x analysis. The same thing can be done to make the pavilion and LG"s align so there is no twist deviation.

A normal Sarin or Ogi scan will not detect this yaw because it does not report the azimuth variations. And the facet sllope or crown and pavilion angles may not have changed.

This speculation furthers what I am saying about azimuth shift and yaw. In such a hypothetical scenario the patterning would be poor as a result.

The result of azimuth shift IS a yawed facet. It may be flat unto itself, but is not flat relative to the axis on which it should have been cut. From the starting point to the end point there will be a difference in the elevation of the facet…Thus it is not flat according to the TRUE idea of where it should lie.

This will show in the patterning.

Your constructive comments are valued. You know, I also have a master cutter in my ear all day long (it itches sometimes).
.