Obstruction vs clustering

[GiveMePerfect]

Hi all, NiceIce has a page about obstruction and it points out the difference between obstruction and clustering. Despite the nice images with arrows, I still cannot tell the difference between the two, and I can't find a single other website that discusses the differences. Can any of the experts here explain why obstruction outside of the arrows is bad but clustering is okay? And how to tell the difference?

 

[John Pollard]

It may be helpful to elaborate on the results of obstruction on a macro level.

In clinical terms, obstruction is neutral. It's not good or bad. It happens. Some amount of light in a diamond's hemisphere will always be obstructed by the presence of a viewer.

The amount of obstruction varies. All things equal, a skinny, bald person obstructs less light than a yoked body builder with a full head of hair. Distance also matters. A diamond held at arm's length will gather more light (less obstruction) than a diamond held close up (more obstruction).

(Image courtesy of AGS)​

With that established, let's talk about contrast pattern, table-reflection - which I have heard referred to as clustering - and leakage.

Contrast Pattern

Obstruction creates a pattern of areas within a diamond which are temporarily dark. This is called the diamond's contrast pattern. A symmetrical, balanced pattern of contrast helps bright areas appear brighter. This works like a chess board, which has only 1/2 the brightness of a sheet of white paper, but appears brighter by comparison.

When moving, a diamond’s contrast becomes a dramatic component of scintillation.

On PriceScope we are used to seeing this effect in diamond photos. This is because the camera itself blocks light from the highest angles - so the lowest facets are darkened, reflecting the black of the camera lens.

Over time, savvy PriceScope vendors have gravitated toward using 30-40 degrees of obstruction in their photos, which also happens to be the range that AGS calculates the average person obstructs at a viewing distance of 25 cm.

This creates a contrast pattern which can be useful for assessment.



Table Reflection

Depending on how a diamond is cut, a side-effect of obstruction is darkness due to table-reflection: Literally, the reflection of the table facet, seen down in the pavilion of the diamond.

• Top examples - Larger tables create more table reflection.
• Center examples - As seen in Ideal Scope.
• Bottom examples - Increasing pavilion depth also creates wider reflections.

Diamond cutters used table reflection to judge pavilion depth for generations; long before precision tools and proportions-scanners existed.

^ Bear in mind these are "perfect" wireframe simulations. In real diamonds, natural variations will skew or bloat the reflection's appearance, creating dark areas which can be asymmetric or chaotic in still photos.

Leakage

Leakage is what it sounds like. Light passes through the diamond and "leaks" from the bottom. That leaves a dark or dead area that is not attributable to obstruction (ergo, not contrast, nor table-reflection). Leakage can be disguised by bright spotlights which essentially overwhelm the darkness. But it shows up in normal lighting - and social environments where most people would rather have that little firecracker at its best.



Summarized

• Obstruction is a constant.
• Bigger people and closer viewing cause more obstruction.
• Well cut diamonds have a nice pattern of contrast which is caused by obstruction in normal viewing.
• Some diamonds can suffer from darkness caused by table-reflection (aka clustering).
• Leakage creates darkness or dead areas not related to obstruction.

Hope it's helpful.

You can see more here: Diamond Performance Explained.

Diamond Performance Explained

Did you know that "fire" and "scintillation" are not the same thing? "Contrast" and "leakage" also differ from each other.

www.pricescope.com

 

[GiveMePerfect]

Wow @John Pollard that is super helpful, thanks so much! I'm deciding between these two diamonds:

and it's been driving me crazy wondering why the one on the left has black at the base of the arrows when its proportions are slightly more ideal than the one on the right, but your info tells me that it's due to the 58% table size. (You can see more about them here if you're interested.)

So does table reflection work like obstruction, in that it will go away if I'm not too close? And is table reflection considered a type of obstruction?

Thanks again!

 

[John Pollard]

You're welcome. I'm happy to help.

So does table reflection work like obstruction, in that it will go away if I'm not too close? And is table reflection considered a type of obstruction?

Yes. But none of the three proportions sets from the thread you linked should have issues caused by table reflection.

and it's been driving me crazy wondering why the one on the left has black at the base of the arrows when its proportions are slightly more ideal than the one on the right...

Don't go crazy. It's not practical to try and draw conclusions about performance components from tweezer photos.

Outside of a structured light environment (like Ideal Scope) or a standardized magnified photo setup - and even those can vary - there are so many variables that can influence what you're seeing. Diamonds are made up of thousands of tiny mirrors. The darkness in a tweezer shot can be something in the environment. The photographer may be standing just a bit closer when the photo on the left was taken. A black bear could have wandered by in the background (okay, that's silly but you get the idea).

Take those images another few times and you could see different patterns. It's why we are big fans of structured light environments around here. They permit a neutral view.

 

[GiveMePerfect]

Yes. But none of the three proportions sets from the thread you linked should have issues caused by table reflection.

Interesting... so looking at the black stuff at the base of the arrows on the left diamond above, would you say that's obstruction if not table reflection? It's also shown in the certificate images so I doubt it's a black bear walking by!

Outside of a structured light environment (like Ideal Scope) or a standardized magnified photo setup - and even those can vary - there are so many variables that can influence what you're seeing.

Unfortunately I've learned that even in a structured light environment, there are still variables! You can see the ASET images and videos of these two diamonds in this thread.

 

[John Pollard]

Unfortunately I've learned that even in a structured light environment, there are still variables! You can see the ASET images and videos of these two diamonds in this thread.

Don't blame that little ASET scope. What you're seeing is irregular setup and weird post-processing. User error, not ASET error. Those who use the tools as intended, with proper setup and approach can produce consistently reliable images.

Interesting... so looking at the black stuff at the base of the arrows on the left diamond above, would you say that's obstruction if not table reflection? It's also shown in the certificate images so I doubt it's a black bear walking by!

As I said above, even magnified setups are subject to environmental confounders.

I think it's the certificate image that's leading you astray: You can see the obstructed pavilion main facet seems to "bend" as it approaches the girdle. I believe the stone was tilted, creating the reflection artifacts - you can see others, lighter, in places like 11:00.



If you look at the other images, taken in SLEs, the pavilion main doesn't have the bent appearance and that artifact is not present.



A conventional H&A viewer has white coming from the highest angles, so it can reinforce contrast pattern interpretation. Here again, pavilion main has no bend.




Does that help?

 

[musicloveranthony]

@OP you’ve been asking some super good questions in your posts. I love reading the responses

 

[GiveMePerfect]

Don't blame that little ASET scope. What you're seeing is irregular setup and weird post-processing. User error, not ASET error. Those who use the tools as intended, with proper setup and approach can produce consistently reliable images.



As I said above, even magnified setups are subject to environmental confounders.

I think it's the certificate image that's leading you astray: You can see the obstructed pavilion main facet seems to "bend" as it approaches the girdle. I believe the stone was tilted, creating the reflection artifacts - you can see others, lighter, in places like 11:00.



If you look at the other images, taken in SLEs, the pavilion main doesn't have the bent appearance and that artifact is not present.



A conventional H&A viewer has white coming from the highest angles, so it can reinforce contrast pattern interpretation. Here again, pavilion main has no bend.




Does that help?

Thanks John! My main concern is the 3.37. I've circled the dark areas I'm wondering about:

Is this table reflection? Obstruction? I find them distracting and I'm wondering if they're just there because the camera was too close?

@OP you’ve been asking some super good questions in your posts. I love reading the responses

Thanks Anthony! Glad my questions are helping

 

[GiveMePerfect]

@John Pollard here's a video of that diamond. I feel like it looks like specs of dirt, or inclusions (I know it's not!).

 

[John Pollard]

Thanks John! My main concern is the 3.37. I've circled the dark areas I'm wondering about:

Is this table reflection? Obstruction? I find them distracting and I'm wondering if they're just there because the camera was too close?

What you have circled is normal table reflection for those proportions. They should only distract you in still photos. In real life viewing a diamond is usually in motion, and those areas become positive contrast elements:

Like the pavilion mains (the arrows) modest TR is dark in this specific, isolated static position. But if you tilt the diamond a degree those areas "turn on," becoming bright, while other areas turn off. Tilt the diamond another degree and those bright & dark areas swap again. Keep the diamond moving and the dynamic "on-off" is what we call scintillation. And it's awesome.

Fun fact, this is why brilliant cuts have different character and charm than step cut diamonds like emeralds and Asschers, which are cut with long, flat facets that don't break the light up so much.

@John Pollard here's a video of that diamond. I feel like it looks like specs of dirt, or inclusions (I know it's not!).

It wouldn't be the needle in a GCAL VS1. Possibly a speck reflecting. More likely something in the environment. I'm distracted by how close the camera is. Pause the video at 15.5 seconds - see how the entire lower half of the diamond is obstructed. Whoever took the video is trying to provide an intimate view of the diamond (which is great). But with the camera that close it's causing more obstruction and reduced brightness than you'd see in normal viewing.

 

[GiveMePerfect]

Yeah, I asked them to send me videos where the distance ranges from 6 inches to 2 feet so I'm hoping to get that today! I would never need my face that close to a 3+ carat diamond anyway