Recommended Proportions

[DejaWiz]

As I was informed by another fellow member according to white flash a 34 crown and 40.6 pavilion angle falls in the “ACA” range. Therefore would this not fit in the premium / ideal category ?

If you are speaking towards the AGA Cut Class chart, then keep in mind (as the creator himself, David Atlas/oldminer, stated) it is of an outdated nature. A 34° CA paired with a 40.6° PA would be perfectly acceptable, albeit on the shallow end of the CA spectrum of what many PS'ers consider the the best pairing for this specific PA.
For me, personally, I would try to target around a 35° CA for a 40.6° PA...personal preference.

 

[Karl_K]

If you are speaking towards the AGA Cut Class chart, then keep in mind (as the creator himself, Davis Atlas/oldminer, stated) it is of an outdated nature. A 34° CA paired with a 40.6° PA would be perfectly acceptable, albeit on the shallow end of the CA spectrum of what many PS'ers consider the the best pairing for this specific PA.
For me, personally, I would try to target around a 35° CA for a 40.6° PA...personal preference.

A true 40.6pa works well with the widest range of realistic CA and table sizes with 77-78 near universal lgf% than any other pavilion angle.

 

[DejaWiz]

A true 40.6pa works well with the widest range of realistic CA and table sizes with 77-78 near universal lgf% than any other pavilion angle.

That's great info, thank you, @Karl_K!
What would be your personal defined CA range for a 40.6° PA with 77-78% LGF?

 

[Karl_K]

That's great info, thank you, @Karl_K!
What would be your personal defined CA range for a 40.6° PA with 77-78% LGF?

It depends on table size and upper girdle angle and were one draws the line on tradeoffs.
So I went with 32 to 40 because its possible to get min. leakage and acceptable brightness in that range with the rest of the facets in ranges that work together with them.

 

[akshayk]

Ah I see so mine is a 34 crown, 40.6 pavilion according to the gia report, 55% table, 61.1% depth.

 

[akshayk]

in fact here is the actual gia certificate what can you tell from this in terms of how it bodes with the aca ?

 

[DejaWiz]

Ah I see so mine is a 34 crown, 40.6 pavilion according to the gia report, 55% table, 61.1% depth.

How does it look and perform to you?

It's GIA EX and darned near AGS 0, so you're good, in my opinion.

 

[akshayk]

It looks great performance I’ve not been able to test yet properly as I haven’t taken it into proper sunlight. But I read an article online where it stated parameters for a super ideal cut and this diamond fitted the 34/40.6 match so brought it based on that lol

 

[DejaWiz]

It looks great performance I’ve not been able to test yet properly as I haven’t taken it into proper sunlight. But I read an article online where it stated parameters for a super ideal cut and this diamond fitted the 34/40.6 match so brought it based on that lol

All the measurements look great...I think you're golden.

 

[sledge]

@DejaWiz and @akshayk the important element being ignored here is GIA gross rounding.

Proportionally nothing is wrong with the stone. My concern would be in the fact the pavilion depth is reported as 42.5 as opposed to 43. Mathematically a 40.6 pavilion calculates as 42.8-ish which would round to 43 depth. The fact it’s being reported as 42.5 concerns me the ACTUAL pavilion angles are on the lower side of 40.6.

To understand that, it goes back to there are 8 actual crown and 8 actual pavilion angles. Each pavilion is not *exactly/perfectly* 40.6. The actuals might be 40.4, 40.6, 40.8, 40.7, 40.3, etc. Whatever the actuals, GIA averages them and then rounds to the nearest 0.2 degrees for pavilion and 0.5 degrees for crown. So if the values averaged were less than 40.6 but got rounded up to 40.6 because of GIA’s gross rounding technique it would make more sense with the 42.5 depth.

We dived into the pavilion but effectively this happens on the crown too. Even LGF has a range. So tour reported 75 LGF could have actuals between 73-77. The low end of 73 would create big fat arrows and more darkness/contrast. With pavilion angles that dip below 40.45 obstruction will be increased and could create an undesirable effect.

It really can’t be stressed enough that actuals matter, along with various lighting conditions and the owners personal preference for fire, scintillation, contrast, etc.

Also remember that 34/40.6 may work in a super ideal but they are looking at a much broader and detailed picture and not being as simplistic as a chart with a few select values on it.

In closing I want to be clear I am not bashing or supporting the stone in question. I think more data is needed to make an accurate assessment. My opinion is it has a very reasonable shot of being a great stone but without additional data it’s a little like playing the lottery looking at GIA proportions only.

 

[akshayk]

Thanks sledge for that! Have you seen the 3 videos I created on YouTube earlier in this thread of the stone ?

 

[DejaWiz]

That's true, @sledge ...and the gross rounding that GIA allows for and applies is one thing that often evades me when when I make recommendations or provide feedback.
Thank you for bringing that up.

 

[sledge]

To add more to my point, this is a stone I found awhile back that had amazing proportions but was a complete dud. Garry disagreed at first but finally came on board as well.

Why Advanced Images Matter...A Bad 34.5/40.8...

I'm just as guilty as anyone. Helping someone find a stone and run across one with that 34.5/40.8 combo. You smile. You then run through the rest of the proportions.... 57 table, 61.7 depth, 75 LGF's, 50 stars and medium-slightly thick girdle....check, check, check,check and check You...

www.pricescope.com

 

[sledge]

Thanks sledge for that! Have you seen the 3 videos I created on YouTube earlier in this thread of the stone ?

You’re welcome and I have not. Life has been chaotic so I’ve been out of pocket lately.

 

[akshayk]

https://youtube.com/shorts/Si5UdVz1oKE?feature=share https://youtube.com/shorts/XwYv3v4H0BY?feature=share https://youtube.com/shorts/tDplbVoruqk?feature=share

 

[sledge]

That's true, @sledge ...and the gross rounding that GIA allows for and applies is one thing that often evades me when when I make recommendations or provide feedback.
Thank you for bringing that up.

I think it’s a hard subject to grasp. The more I have learned the more I lean towards “it depends”, lol. Many times we can see tell-tale signs but videos and advanced images really help provide assurance to online buyers.

And IMO, the images (or use of scopes) help in-person buyers too. Most people don’t have a trained eye and jewelers notoriously blast their customers with lighting that makes frozen spit look great. The game is rigged against average Joe IMO.

Diamond education is so badly needed in our world but honestly labs and some jewelers have quite a bit to lose by an educated consumer so I expect growth to go at a snails pace.

 

[DejaWiz]

I think it’s a hard subject to grasp. The more I have learned the more I lean towards “it depends”, lol. Many times we can see tell-tale signs but videos and advanced images really help provide assurance to online buyers.

And IMO, the images (or use of scopes) help in-person buyers too. Most people don’t have a trained eye and jewelers notoriously blast their customers with lighting that makes frozen spit look great. The game is rigged against average Joe IMO.

Diamond education is so badly needed in our world but honestly labs and some jewelers have quite a bit to lose by an educated consumer so I expect growth to go at a snails pace.

I hear ya. Almost all of the sales associates my wife and I visited early spring when we started shopping gave me the ole clueless blinking/deer in headlights look when I asked if they had an IS and/or ASET that I could use. After explaining to them that they are simple yet effective tools for evaluating symmetry and light leakage, most of them would simply respond with some along the lines of "well, the report shows that it's already excellent/ideal, so you won't need that because they've done the work for you."

 

[Karl_K]

The more I have learned the more I lean towards “it depends”, lol.

Yep the more you know the more often the best answer is "it depends"

 

[Garry H (Cut Nut)]

When I established HCA my best perfomance proportions were 34 57% 40.6 (as best I remember as it was 21 years ago).
The reason was that this compensated for the back then bigger variations in symmetry - especially the pavilion angle.
Back then GIA placed a higher value on meet point human grading. Today symmetry is based on scan results and while the deviations can be bigger than I would like - especially in the pavilion, and the flawed way that GIA measure pavilion angle variations even 15 years after we published the OctoNus theory:

Срок регистрации домена закончился

 

[akshayk]

I’ve ordered a blue scope heart and arrow on eBay so will take some pics of that soon for you all. But when I used the toilet roll as a scope I saw some very nice and clear arrows actually which gives me good confidence my stone is a good cut. I await to see it through that heart and arrow scope. Hopefully I’ll be able to use it with my stone being already mounted to my ring. I know I won’t be able to see the hearts but the arrows should be clear to see.

 

[sledge]

@akshayk i saw your videos and original thread. I will make some additional comments in that thread so we don’t piggy back off Garry’s thread who is talking philosophy more than a specific sample case like yours.

I previously responded because I believe GIA gross rounding has to be considered when pushing a chart of recommendations or any GIA stone really.

 

[akshayk]

@akshayk i saw your videos and original thread. I will make some additional comments in that thread so we don’t piggy back off Garry’s thread who is talking philosophy more than a specific sample case like yours.

I previously responded because I believe GIA gross rounding has to be considered when pushing a chart of recommendations or any GIA stone really.

@sledge hey, okay @ me with your response I haven’t seen anything on that post come up just yet. Would be keen to see what you have to say.

 

[akshayk]

Hi all happy Monday. After looking at my diamond through the hearts and arrow scope I can confirm it is a super ideal cut. All arrows are proportional as per pictures I’ve seen online of a super ideal cut with contrasts being very strong. Wow I love this scope!

 

[Garry H (Cut Nut)]

Hi all happy Monday. After looking at my diamond through the hearts and arrow scope I can confirm it is a super ideal cut. All arrows are proportional as per pictures I’ve seen online of a super ideal cut with contrasts being very strong. Wow I love this scope!

Sorry, but there is way more to a super ideal cut than simply seeing an arrows pattern.

 

[akshayk]

Thoughts? This is my ring mounted using a arrow scope blue filter

 

[lovedogs]

Thoughts? This is my ring mounted using a arrow scope blue filter

please either start a new thread or post on your original thread, as this one isn't for specific questions about one stone.

 

[akshayk]

please either start a new thread or post on your original thread, as this one isn't for specific questions about one stone.

Sorry I will post on another thread

 

[Garry H (Cut Nut)]

Sorry I will post on another thread

After closely examining your image and discussion my thoughts are inside these brackets ( ).

 

[John Pollard]

Dear all,

1. There has been great interest in the visual tendencies chart @Garry H (Cut Nut) posted for input. Thanks to all who offered comments. Very helpful.

2. Inside Garry's wide ranging numbers there is been a more conservative proportions range veteran members have recommended for years - to shoppers seeking "safe" results (i.e. high marks in any grading system) and balanced performance.

For your convenience, I've staged both pieces of information together with added context, usage notes and limitations.

Diamond Proportions page

Diamond Proportions

The physics of light-behavior within a diamond are reliably consistent, which means proportions measurements can be useful.

www.pricescope.com

Appreciation to @Karl_K, with whom Garry and I consulted.

Big cheers to @prs , @DejaWiz , @sledge , @flyingpig , @diamondseeker2006 , @OohShiny , @Mlh and other consumers who took time to weigh in - here and in the other thread.

Gracious thanks to the reliable pros who weighed in as well. You know who you are.

We hope the new page is useful.

 

[DejaWiz]

Dear all,

1. There has been great interest in the visual tendencies chart @Garry H (Cut Nut) posted for input. Thanks to all who offered comments. Very helpful.

2. Inside Garry's wide ranging numbers there is been a more conservative proportions range veteran members have recommended for years - to shoppers seeking "safe" results (i.e. high marks in any grading system) and balanced performance.

For your convenience, I've staged both pieces of information together with added context, usage notes and limitations.

Diamond Proportions page

Diamond Proportions

The physics of light-behavior within a diamond are reliably consistent, which means proportions measurements can be useful.

www.pricescope.com

Appreciation to @Karl_K, with whom Garry and I consulted.

Big cheers to @prs , @DejaWiz , @sledge , @flyingpig , @diamondseeker2006 , @OohShiny , @Mlh and other consumers who took time to weigh in - here and in the other thread.

Gracious thanks to the reliable pros who weighed in as well. You know who you are.

We hope the new page is useful.

This is an absolutely beautiful update to the educational section! Kudos and cheers!