Obstruction

[FB.]

I'm interested in an unusual stone (because it might have an interesting character) that has been suggested might have significant obstruction issues.
Having used the search function for a number of hours, I am mostly finding things that I already knew, but I'm struggling to envisage the effect on a badly obstructed stone.

I appreciate that it might have a very dark set of arrows under the table when viewed straight-on (even as a pendant) and I have no problems with that. I call it "character".

But might the stone suffer other adverse effects that explains why GIA don't extend their Excellent cut grade to such unusual angles?
Could the obstruction be so bad that the entire stone is black and dead in almost any lighting condition?
Can the stone look good from any viewing angle at all?
Will the obstruction mean that although the angles might be good, I'd only enjoy the stone's beauty if I had a lamp mounted on my forehead to actually give the stone some light to sparkle with that my head would otherwise block?

Basically, I'd really appreciate some more info on badly obstructed stones and their performance, since the chances of images for diamonds - in the UK - is virtually zero. Either I have to view in person, or buy it and return, or let a potentially good interesting stone pass by.

However, I am so confused about how bad it might look that I'm tempted to just buy it, so that I can describe - first-hand - the effect of severe obstruction to newcomers!

The details are below for the stone that sparked this topic:

GIA cert details:
Round brilliant
Cut grade: VG
Polish: EX
Symmetry: EX
Table: 55%
Crown angle: 38.5'
Crown height: 18.0%
Star: 50%
Girdle: thin-medium (faceted)
Pavilion depth: 41.5%
Pavilion angle: 39.8'
Lower half: 75%
Total depth: 62.9%


Thanks,

 

[Lorelei]

FB, have you seen Garry''s video titled Head Obstruction? That gives a good idea of what it looks like.

http://diamondscope.pricescope.com/

 

[outatouch0]

Darn, it says "no video" - anyone else having problems viewing?

 

[FB.]

Yes, I've seen Garry's obstruction video.
But most talk of obstruction is referring to perfect face-on views of the diamond. But diamonds are rarely viewed face-on at 6-12 inches, when being worn, particularly to casual observers.
I'm curious how a badly obstructed stone would perform in a variety of normal environments, when illuminated by various types of light sources and viewed from a variety of angles.

In this example, this stone would be a pendant, so the minimum viewing distance would be about 2ft and the viewing angle could be as much as 30 degrees either side of straight-ahead.

Basically, if the viewing angle and lighting conditions change, is the stone always going to have serious problems no matter what? Is obstruction always going to starve it of enough light to sparkle?

 

[stone-cold11]

Yap, obstruction issue depends on the viewing distance, the further you head is from the stone the less shadow you will throw onto the stone. Exact distance for this configuration I am not sure, probably Karl can simulate for you.

 

[FB.]

Yes, Karl created an approximation of the stone at 2ft.
But a picture can only represent one distance, one viewing angle and one light condition at a time.

This diamond would be considered to have a bias towards fire, although HCA suggests good all-round light-handling. In my experience, fire is best seen in spotlight-type environments at a few feet distance.

 

[Karl_K]

Date: 11/17/2009 5:25:24 PM
Author: FB.
Yes, I''ve seen Garry''s obstruction video.

But most talk of obstruction is referring to perfect face-on views of the diamond. But diamonds are rarely viewed face-on at 6-12 inches, when being worn, particularly to casual observers.

I''m curious how a badly obstructed stone would perform in a variety of normal environments, when illuminated by various types of light sources and viewed from a variety of angles.


In this example, this stone would be a pendant, so the minimum viewing distance would be about 2ft and the viewing angle could be as much as 30 degrees either side of straight-ahead.


Basically, if the viewing angle and lighting conditions change, is the stone always going to have serious problems no matter what? Is obstruction always going to starve it of enough light to sparkle?

I showed you the simulation for that one at 25 inches so it will still be an issue in a pendant.
What is does is make the table look dark which is different than when the arrows show because it is just under the table not the arrow heads.
It also shows for a range of tilt in that combo.

 

[Karl_K]

lay a powdered sugared donut on a black piece of paper and that is how it will look.

 

[Rockdiamond]

Interesting thread fb!!!

I''m very interested in the issue of "obstruction"
What if we''re looking at a diamond in the sun.
If it''s in the sun, it''s getting direct light.
If your head is blocking it, it''s in the shadow.
That''s a clear cut case of "obstruction- does it pertain to the "Head obscuration ( or obstruction) discussed here?
But, in reality how many times will a diamond be positioned so that the head can block all the ambient light- which also plays a huge role in how we see things.
It would seem to be rare that the head would be in a position to "obstruct" most light from hitting the diamond unless you''re using a well focused spotlight- or maybe a laser.
Otherwise ambient light will flood top corners, and table.

 

[stone-cold11]

David, you still have no idea how optics work....

 

[Rockdiamond]

Interesting concept.
I do know how actual diamonds work, which has proved to be quite sufficient over the years.


Stone, if you can answer the questions I''m asking, I''d be interested in learning about what you are refer to as "optics"

Sun shining on diamond, no obstruction.
Head blocking diamond, sounds like obstruction, no?

In the second case, how much light is getting into that diamond, and from what direction?
Could the head block only the table?
Are ambient rays reaching the top corners and somehow missing the table?

What is the specific lighting condition that allows "obstruction"?
How common is that particular lighting scenario?

Would understanding what looks good in a diamond be sufficient, or does one need to figure out how the light travels through the diamond to be able to judge it''s characteristics in person?

 

[Gleam]

Date: 11/18/2009 5:06:27 PM
Author: Rockdiamond

Sun shining on diamond, no obstruction.

Head blocking diamond, sounds like obstruction, no?


In the second case, how much light is getting into that diamond, and from what direction?

Could the head block only the table?

Are ambient rays reaching the top corners and somehow missing the table?


What is the specific lighting condition that allows 'obstruction'?

How common is that particular lighting scenario?


Would understanding what looks good in a diamond be sufficient, or does one need to figure out how the light travels through the diamond to be able to judge it's characteristics in person?

I'm somewhat fuzzy on this concept and interested in knowing as well. If I recall correctly, someone here told me my 2.5 carat RB would suffer from a case of head obstruction. I've tried to figure out exactly what this means myself -- putting my diamond at a distance, then yet closer, etc. etc.

My living room and bathroom are the two places I've tried this and it must have the incorrect type of light to see this effect, or else I'm just too dense to notice it. I don't see any substantial darkening when I bring my diamond closer to my eye.

This is a subtle concept that I think is pretty lost on the average layman if not the average PSer. I'm an average layman for sure, because I don't think this has nor will it in the future come up as one of my concerns. In real life, my hand is gesturing as I speak or cutting my steak or reaching out to pet a dog -- it's not at any one set point where I would personally bother worrying about head obstruction. It surprises me that this is a big deal for some people but again, I am a total layman. I would like to understand why this effect is considered so awful, however. The lack of sparkle if it's close to your head?

 

[FB.]

So...maybe one of the experts can answer because I'm sure that a lot of PS-ers would love to know the implications of a stone with obstruction - just as they are keen to know about steep/deep, which seems to be fretted about a lot.

Does a stone with obstruction never sparkle because your head will always block the light source in and out, for that set of angles?
...or does it always have a dead middle - a bit like an inverse steep/deep?

Does the stone have any hope of sparkling when viewed from slight angles, or is the sparkle always directed perfectly straight out of the top (where your head would block the light source)?

With this stone seemingly having a bias towards fire, how might such a stone perform in the lighting environments that show the most fire?

A big thanks to anyone who can explain how an obstructed stone behaves in a variety of light conditions and angles.

 

[John P]

Date: 11/19/2009 10:59:38 AM
Author: FB.

So...maybe one of the experts can answer because I'm sure that a lot of PS-ers would love to know the implications of a stone with obstruction - just as they are keen to know about steep/deep, which seems to be fretted about a lot.

"Obstruction" aka "Head Shadow" (Bruce Harding) or "Obscuration" (AGS) is not bad. In fact without it a diamond with great light return would appear like a headlight.

At any angle the viewer's head will block some light getting to the diamond (unless you're the invisible woman). This causes parts of it to darken. Diamonds with a pleasant balance of light return and obstruction through a range of tilt have positive "contrast" (AGS term) between light and dark areas. As the diamond is tilted the light and dark areas reverse themselves and the on-off effect related to the contrast pattern is what we call scintillation. This is part of the AGSL ray-tracing evaluation. GIA calls the same thing "pattern" and it is part of their human observation-evaluation.

Diamonds with shallow pavilions, coupled with certain crowns, will pick up more of your head's shadow, especially as you move closer to the diamond. If you look at this HCA page you will see a graphic indicating "young people rings," "older people rings," and "pendant and earring stones." This notion follows the cutters line relative to obstruction: Younger rings show the least obstruction (presuming young people can focus closer and will look more closely at their rings), older show slightly more (close-up vision isn't as good in most elders) and pendant/earring stones gather enough obstruction that Garry recommends they be used in capacities where head shadow has less effect...for example most people don't "obstruct" a pendant diamond with their heads - unless they are getting pretty fresh.

Does a stone with obstruction never sparkle because your head will always block the light source in and out, for that set of angles?
...or does it always have a dead middle - a bit like an inverse steep/deep?

A configuration with too much obstruction darkens the stone as the observer moves closer, even in strong lighting. GIA has less tolerance for shallow stones because they utilized a 45 degree obstruction model in human observations (which translates to a pretty close viewing distance). AGS tests both 30 and 40 degrees in theirs. This means diamonds with obstruction problems are more readily penalized in GIA's metric and deeper stones are somewhat tolerated since steep/deeps suffer from leakage rather than obstruction.

Does the stone have any hope of sparkling when viewed from slight angles, or is the sparkle always directed perfectly straight out of the top (where your head would block the light source)?

The "normal" assessment position is looking straight-down on the diamond, and some metrics utilize a range of tilt. Generally around 15 degrees to check for fish-eye/girdle reflection issues. Sometimes more (AGS Platinum utilizes a 45 degree range of tilt). There are some shapes which, in my opinion, look dynamite viewed from the side where the obstruction pattern is comepletely unconventional. For the traditional RB I am afraid we are locked into a crown-down and slight tilt pattern of assessment...getting into side-views could be fun but it will be like visiting another planet.

With this stone seemingly having a bias towards fire, how might such a stone perform in the lighting environments that show the most fire?

Interesting question. Dispersion is a function of pavilion/critical angle + crown configuation + visibility of VFs. I just looked and see fire nicely in my near-Tolk to about 30 degrees of tilt, with a (relatively) high crown and midsize lower halves. As mentioned above, we tend to observe diamonds face-up rather than from the side for performance...although we all "play" with them non-officially now and again. It would be interested in hearing others' experiences and/or modeling of unconventional viewing angles if someone has time.

A big thanks to anyone who can explain how an obstructed stone behaves in a variety of light conditions and angles.

I may not have even come close - but hope the thoughts help.

 

[stone-cold11]

Date: 11/18/2009 5:06:27 PM
Author: Rockdiamond
Interesting concept.

I do know how actual diamonds work, which has proved to be quite sufficient over the years.

Stone, if you can answer the questions I'm asking, I'd be interested in learning about what you are refer to as 'optics'

Sun shining on diamond, no obstruction.
Head blocking diamond, sounds like obstruction, no?

In the second case, how much light is getting into that diamond, and from what direction?
Could the head block only the table?
Are ambient rays reaching the top corners and somehow missing the table?

What is the specific lighting condition that allows 'obstruction'?

How common is that particular lighting scenario?

Would understanding what looks good in a diamond be sufficient, or does one need to figure out how the light travels through the diamond to be able to judge it's characteristics in person?

Already explained by Serg in the steep/deep thread.

 

[Lorelei]

Great post John!

 

[Michael_E]

Obstruction is probably not going to be a problem with this diamond if it''s used as a pendant. The reason being that most of the light entering the diamond will be from above and not from behind the viewers head, (as it would be if it were being viewed in a ring setting with the table pointing up and the viewers head above the stone. The best modeling for this type of situation would be to have the light entering the stone from maybe 45° off the perpendicular and the viewer being at maybe 30° off the perpendicular to the table. Just a guess, but I think that this high table design would work really nicely as a pendant. Modeling in a more normal fashion would have the stone looking dark, but not many people are going to bend down to look directly at the stone, and from a distance the head obstruction would have no effect, (although the high crown angles will have this looking quite fiery at a distance).

 

[Rockdiamond]

Very informative post John!
Hearing a reasonable explanation of what "Obstruction" refers to is extremely useful to this conversation. Thank you very much for adding it!

Stone, if this was explained in the "steep deep thread" I somehow missed that part.
If you could please copy and paste the explanation from that thread in here, I''d appreciate it.

It seems to me that the term the term, or concept of "Obstruction" is used frequently here on PS to create doubt about a stone''s beauty.

There seems to be a lot of holes in the entire theory.
At the root of my problem with how this translates into reality is this statement: "at any angle the viewers head will block some light getting to the diamond"
You would not need to be a member of the Fantastic Four to look at a diamond in a position that your head blocks no light.

I would again use the example of a diamond in the sunlight.
Either it''s obstructed, or in the sun.
If it is in the sun, which rays is the viewer''s head blocking?
It makes sense to me that if the stone is in the sun, the head could only be interfering with ambient light.

Same question about looking at a diamond under a grading lamp.
I''m behind the diamond which is directly under the lamp.
How can my head obscure an important amount of light- enough so that it would have any effect against the bright diamond light which is 3 inches directly above the diamond?

Taken one step further, I know that a deeply cut diamond will show a dark center if I''m looking at it under a diamond light- where the head can not obstruct any light. Therefore "head obstruction" is not causing the center of the deep diamond to look dark.

There would seem to be so few positions of light and diamond that would allow the head to obscure significant light.

The theory that the head''s shadow will adversely affect one diamond more than another, based on the crown and pavilion angle might make academic sense, but it would seem to be hard to correlate this to "real world" examples when people look at diamonds. This is partially because of how many different lighting situations we are all exposed to all day long.

 

[Karl_K]

Any optical symmetrical RB except some very deep ones will look like this if you get it close enough to your face.
The distance it begins and ends to look like this is the range of obstruction we are concerned about.
At close range this can provide and enjoyable pattern like in this image.

 

[Karl_K]

but if it stays that way to far out beyond the point that the patterns is visible it just looks dark.
This is said to have obstruction issues.
This point is general considered to be 1/2 arm length for most people or about 15 inches.

 

[Karl_K]

Here is one with no issues, subtle contrast and nice brightness.

 

[rockabee]

Date: 11/19/2009 12:21:01 PM
Author: John Pollard

Date: 11/19/2009 10:59:38 AM
Author: FB.

So...maybe one of the experts can answer because I''m sure that a lot of PS-ers would love to know the implications of a stone with obstruction - just as they are keen to know about steep/deep, which seems to be fretted about a lot.

''Obstruction'' aka ''Head Shadow'' (Bruce Harding) or ''Obscuration'' (AGS) is not bad. In fact without it a diamond with great light return would appear like a headlight.

At any angle the viewer''s head will block some light getting to the diamond (unless you''re the invisible woman). This causes parts of it to darken. Diamonds with a pleasant balance of light return and obstruction through a range of tilt have positive ''contrast'' (AGS term) between light and dark areas. As the diamond is tilted the light and dark areas reverse themselves and the on-off effect related to the contrast pattern is what we call scintillation. This is part of the AGSL ray-tracing evaluation. GIA calls the same thing ''pattern'' and it is part of their human observation-evaluation.

Diamonds with shallow pavilions, coupled with certain crowns, will pick up more of your head''s shadow, especially as you move closer to the diamond. If you look at this HCA page you will see a graphic indicating ''young people rings,'' ''older people rings,'' and ''pendant and earring stones.'' This notion follows the cutters line relative to obstruction: Younger rings show the least obstruction (presuming young people can focus closer and will look more closely at their rings), older show slightly more (close-up vision isn''t as good in most elders) and pendant/earring stones gather enough obstruction that Garry recommends they be used in capacities where head shadow has less effect...for example most people don''t ''obstruct'' a pendant diamond with their heads - unless they are getting pretty fresh.

Does a stone with obstruction never sparkle because your head will always block the light source in and out, for that set of angles?
...or does it always have a dead middle - a bit like an inverse steep/deep?

A configuration with too much obstruction darkens the stone as the observer moves closer, even in strong lighting. GIA has less tolerance for shallow stones because they utilized a 45 degree obstruction model in human observations (which translates to a pretty close viewing distance). AGS tests both 30 and 40 degrees in theirs. This means diamonds with obstruction problems are more readily penalized in GIA''s metric and deeper stones are somewhat tolerated since steep/deeps suffer from leakage rather than obstruction.

Does the stone have any hope of sparkling when viewed from slight angles, or is the sparkle always directed perfectly straight out of the top (where your head would block the light source)?

The ''normal'' assessment position is looking straight-down on the diamond, and some metrics utilize a range of tilt. Generally around 15 degrees to check for fish-eye/girdle reflection issues. Sometimes more (AGS Platinum utilizes a 45 degree range of tilt). There are some shapes which, in my opinion, look dynamite viewed from the side where the obstruction pattern is comepletely unconventional. For the traditional RB I am afraid we are locked into a crown-down and slight tilt pattern of assessment...getting into side-views could be fun but it will be like visiting another planet.

With this stone seemingly having a bias towards fire, how might such a stone perform in the lighting environments that show the most fire?

Interesting question. Dispersion is a function of pavilion/critical angle + crown configuation + visibility of VFs. I just looked and see fire nicely in my near-Tolk to about 30 degrees of tilt, with a (relatively) high crown and midsize lower halves. As mentioned above, we tend to observe diamonds face-up rather than from the side for performance...although we all ''play'' with them non-officially now and again. It would be interested in hearing others'' experiences and/or modeling of unconventional viewing angles if someone has time.

A big thanks to anyone who can explain how an obstructed stone behaves in a variety of light conditions and angles.

I may not have even come close - but hope the thoughts help.

i am very new to all of this, but it seems to me that most real world views of a diamond are at a tilt. sure, sometimes a person looks directly at it so that they are facing the table at a bird''s eye view. but even in those cases, it is not clear that primary light sources are directly obstructed. anyways, none of this is a revelation. but what are the implications for the importance of viewing light return at a tilt and what are the ramifications for instruments like the ideal scope and aset, which really measure the view of light return when viewed directly above the center of the table?

should there be a tilted version of an aset/IS made to complement the existing ones so that we can have a sense of light performance at other common viewing angles? i think AGS measures performance on their DQD with many angles of viewing and DQR with a bird''s eye and a 45 deg angle (don''t quote me on that). are we overly obsessing on every nook and cranny of IS/aset images when in fact, that view may represent only a limited % of total real life viewing situations?

 

[Karl_K]

Now the second consideration is does the obstruction go away with tilt?

This one has mild issues.

 

[Karl_K]

same one with mild tilt, since in the real world lighting is often off axis and mixed this stone would show no real issues in the real world.

 

[John P]

Date: 11/19/2009 3:32:17 PM
Author: rockabee

i am very new to all of this, but it seems to me that most real world views of a diamond are at a tilt. sure, sometimes a person looks directly at it so that they are facing the table at a bird''s eye view. but even in those cases, it is not clear that primary light sources are directly obstructed. anyways, none of this is a revelation. but what are the implications for the importance of viewing light return at a tilt and what are the ramifications for instruments like the ideal scope and aset, which really measure the view of light return when viewed directly above the center of the table?

should there be a tilted version of an aset/IS made to complement the existing ones so that we can have a sense of light performance at other common viewing angles? i think AGS measures performance on their DQD with many angles of viewing and DQR with a bird''s eye and a 45 deg angle (don''t quote me on that). are we overly obsessing on every nook and cranny of IS/aset images when in fact, that view may represent only a limited % of total real life viewing situations?

The AGS Platinum (DQD) metric models the diamond in 3D through a 45-degree range of tilt, in 1 degree increments, at both 30 and 40 degrees of obstruction to arrive at its deductions/values. The Gold uses static 2D measurements. They do incorporate predictive tilt to some degree but it does not take the specific diamond into account.

In the trade this has been discussed ad nauseum. The consensus from the major labs and researchers is that analysis face-up is the logical standard approach. It was the basis for the Firescope, Gilbertson, Ideal-Scope and ASET. It is how GIA conducted their observational research.

The most advanced studies are now accounting for tilt as well, which has allowed more in-depth assessment approaches and improved handshaking between diamond design/modeling and production.

 

[Lorelei]

Date: 11/19/2009 3:28:36 PM
Author: Karl_K
but if it stays that way to far out beyond the point that the patterns is visible it just looks dark.
This is said to have obstruction issues.
This point is general considered to be 1/2 arm length for most people or about 15 inches.

" The Donut Effect ".

 

[Rockdiamond]

Date: 11/19/2009 3:28:36 PM
Author: Karl_K
but if it stays that way to far out beyond the point that the patterns is visible it just looks dark.
This is said to have obstruction issues.
This point is general considered to be 1/2 arm length for most people or about 15 inches.

Karl- actual photos that show the darkness you created on this simulation of a diamond are what I''ve been asking to see.
Your gnerated image shows it clearly. If any of the actual diamonds being knocked showed such a clear dark pattern, than it would be easy to illustrate a problem with a camera.

John- I''d be very interested in your opinion of all this.
Aren''t there many positions where the head can not obstruct light?

 

[rockabee]

Date: 11/19/2009 3:40:17 PM
Author: Karl_K
same one with mild tilt, since in the real world lighting is often off axis and mixed this stone would show no real issues in the real world.

this is precisely what i am referring to regarding how different things can be with tilt.

 

[stone-cold11]

This post.
https://www.pricescope.com/idealbb/dview.asp?postID=2184248&mtype=reply

 

[Rockdiamond]

Stone, here''swhat you''re referring to as an explaination:

Some diamonds could be bright only if your head is Bright(Bright light source, I mean). You can use a lot of lights in different directions but you will see just yourself in these diamonds. most known example is 90 degree prism. but you can achieve same result by diamonds too. See example. most known example for diamond traders is Bow-tie Sergey Sivovolenko
CEO OctoNus

I have tremendous respect for Sergey- yet this post does not explain "Obstruction"as it''s commonly used on PS.