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OEC Ideal Cut?

prs

Brilliant_Rock
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I should point out that for an MRB the lower half facet length is calculated as a % of the length of the pavilion from girdle to point. However an OEC doesn't have a point, and the length of the pavilion will vary depending upon the size of the culet. So if, for example, the lowers on an MRB calculate out to 60%, those same lowers will calculate out to 65% on an OEC with a 5% culet, and 70% on an OEC with a 10% culet.

To remain consistent the %Lowers given in my facet diagrams above are calculated as if the diamond had a point.

I don't think GIA figured this into their 60% max lowers cut criteria, but I suspect they might be making allowances for it. I see a whole bunch of stones with lowers obviously greater than 60% that have been given the OEC designation.
 
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LightBright

Brilliant_Rock
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I've owned a perfectly cut August Vintage Cushion and I sold it, it was pretty but I liked the other diamonds I have more than it..... I now have a fat arrowed transitional that is a fireball that has a table you could drive a bus through, a 100+ year old slightly wonky 5.34 carat OEC and a really cool genuine Antique stone that is a cross between a French cut, a Peruzzi Cut, and an Old Mine cut.

Genuine Antique stones have been handcut often without the use of modern day cutting machines and techniques, they therefore should be judged with your eyes NOT with a number chart.

If you go for a new old cut that is another way to get perfection if you don't want to spend time searching for the right stone for you.
I agree with this 100%. I’d love to see your old cuts. Do you have SMTB on each of these? I’ll search but search seems harder (often fruitless) than I thought it would be.
 

LightBright

Brilliant_Rock
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This thread is awesome. Thank you PRS for starting it and thank you to the esteemed posters posting. So much to think about and explore.

PRS I agree with you on being mindful of the technical parameters of beauty. Aside from that perfection I’ve also come to appreciate the artistry of “imperfect” (eg non-symmetric, manual not mechanized) cutting, being able to see color changes in various lightings, being able to appreciate the visual clarity and inclusions of the rough, the girdle which sometimes contains parts of the actual skin of the diamond, and the 3D dimensionality of the cut diamond, versus the top down characteristics. I have several antique diamonds and i know which of them to wear in which lighting for maximum performance. When my OEC falls flat, I can count on my OMB to look amazing. The technical perfection of cut is a worthy pursuit, but other things are also important factors in selecting and loving an antique diamond. Just an aside...
 
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arkieb1

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I agree with this 100%. I’d love to see your old cuts. Do you have SMTB on each of these? I’ll search but search seems harder (often fruitless) than I thought it would be.

 

prs

Brilliant_Rock
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This thread is awesome. Thank you PRS for starting it and thank you to the esteemed posters posting. So much to think about and explore.

PRS I agree with you on being mindful of the technical parameters of beauty. Aside from that perfection I’ve also come to appreciate the artistry of “imperfect” (eg non-symmetric, manual not mechanized) cutting, being able to see color changes in various lightings, being able to appreciate the visual clarity and inclusions of the rough, the girdle which sometimes contains parts of the actual skin of the diamond, and the 3D dimensionality of the cut diamond, versus the top down characteristics. I have several antique diamonds and i know which of them to wear in which lighting for maximum performance. When my OEC falls flat, I can count on my OMB to look amazing. The technical perfection of cut is a worthy pursuit, but other things are also important factors in selecting and loving an antique diamond. Just an aside...
I agree with everything you say. The problem I had when I started this thread is I had no idea what to look for in a beautiful OEC. Sure I had reached the point of knowing one when I saw one, but the idea of ordering and returning OEC after OEC until we found "the one" definitely did not feel right to me.

I wanted to do the research necessary to narrow the field, so I could look at a vendor photo and know enough to say this one is a definite maybe. It's a five hour round trip drive for us to go to LA, and that's if we avoid rush hour. I needed to be able to communicate with OEC dealers and explain exactly what we were looking for, so that trips would be productive and not a waste of time and energy.

I think I know now what we are looking for with regard to the minor facets. It's a stone that combines August Vintage aligned table and star facets with Victor Canera's fabulous flower petal pattern! :eek-2: It would be fantastic if we could find a hundred year old, gorgeous old cut with all the idiosyncrasies you describe. For sure that's what we are looking for, and it would give DW two wonky old Europeans to play with! :lol-2: Unfortunately OECs in our size and color are few and far between, so my fall back position is a re-cut, and I need to know the numbers to have any chance of doing that successfully.

I'm still on my learning curve and appreciate you asking @arkieb1 to add her outstanding diamonds here. I hadn't come across the Charlie thread before, and it's an eye opener to me that a stone with a CA of 37.5°, a PA of 41.0°, and a table of 60% could produce an OEC with exactly the light performance I'm looking for.

To me, keeping an open mind, and learning new things adds the spice to life. For sure, there's a lot of spice to be found in old cut diamonds!
 
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LightBright

Brilliant_Rock
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Hi PRS. I’m curious about a couple things. What are your color and size/dimension preferences. (Just curious.). Also, is there someone you have in mind who can actually recut an existing diamond to the type of antique style diamond you prefer? We are getting into the top secret diamond secret handshake world with discussing specifics here, but I am curious about your plan, considering that you have VERY specific cut preferences, and finding a specific cut along with color and dimensions have not yet come into play in the search process yet. As most people who have found antique stones will tell you, compromise along one of those parameters is often necessary.
 

prs

Brilliant_Rock
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Al Gilbertson's book "The American Cut" is mostly about how table size, and crown and pavilion angles, changed over the years in a progression that lead us to the modern round brilliant. However there are nuggets of information in the book that give us clues about the development of the most beautiful "OECs". Historically lower half facets, also known as lower girdle facets, were cut at 30%, and it is the American Henry Morse, the first cutter in the world to cut for beauty rather than weight, who is credited with extending the length of the lower halves. Here's an excerpt from the book.

The American Cut Pg 105.png

Morse cut the Tiffany II diamond in 1883 and it is the longer lower halves that produced the Kozibe effect, a reflection of the culet in the crown bezel facets. This diamond also exhibits the flower petal pattern, so with a 46% table I figure the lowers must have been at least 65%. The Tiffany II is a very historic diamond given it is likely the first ever known to exhibit both the Kozibe effect and the flower petal pattern. The whereabouts of this diamond are unknown, but if it ever resurfaces I would love to see it in person.
 

prs

Brilliant_Rock
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Interestingly the book goes on to say that smaller tables and the Kozibe effect were very much a feature of American cut diamonds.

The American Cut Pg 106.png

I guess what surprised me the most was that many of the effects we associate with the most beautiful old "Europeans" actually originated in America.
 

prs

Brilliant_Rock
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Hi PRS. I’m curious about a couple things. What are your color and size/dimension preferences. (Just curious.). Also, is there someone you have in mind who can actually recut an existing diamond to the type of antique style diamond you prefer? We are getting into the top secret diamond secret handshake world with discussing specifics here, but I am curious about your plan, considering that you have VERY specific cut preferences, and finding a specific cut along with color and dimensions have not yet come into play in the search process yet. As most people who have found antique stones will tell you, compromise along one of those parameters is often necessary.
We are looking for an L VS2 with a spread of at least 10.50mm, so that means 4.5ct plus. Figuring out the minor facet patterns we like was easily done from photos. Figuring out what crown and pavilion angles make for the most beautiful OECs is going to be more difficult. The fact GIA doesn't put these angles on their OEC certs just makes it even more difficult.

A stone this size isn't going to be cheap and it's going to be DW's forever diamond. I want to know all the technical stuff so we can make an educated purchase. I'll cross the re-cut bridge when we come to it, and hopefully we'll never come to it, but if we do I want to be knowledgeable enough to have an intelligent conversation with the cutter.
 

prs

Brilliant_Rock
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I just came across a terrific video by Jonathan Weingarten, @Rhino, made when he was with Good Old Gold. Jonathan developed the August Vintage line of Old Cuts. The video describes in great detail, and in different lighting conditions, how the lower half facets influence the light reflection pattern of OECs as compared to MRBs. If you look closely you'll also be able to pick out the flower petal pattern in the first OEC.

 
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prs

Brilliant_Rock
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In the book "The American Cut" there is some interesting information about the importance of the culet in old cuts.

The American Cut Pg 182.png

In those days the culet actually improved light performance. However I checked the trigonometry of the stone in Fig, a-5 and it has a crown angle of 50° and a pavilion angle of 53°. I don't think DW would be happy with one of those!!!

For diamonds cut with modern day angles it appears the culet is a detriment to light performance rather than a help. For a diamond with ideal proportions GIA will only give an Excellent cut grade to a diamond with a small culet. A medium culet drops the grade to Very Good, slightly large and large culets drop the grade to Good, and a very large culet drops the grade to Fair.

Quite why GIA insist an OEC must have a slightly large or larger culet is beyond my understanding when their own grading system penalizes those very same culets.

I think I understand now why Jonathan decided that AVRs would have no culet, and Victor's CERs only have a medium culet. In terms of finding the ideal OEC I think I'll be on the lookout for a stone with a culet no bigger than slightly large.
 

prs

Brilliant_Rock
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We all know that GIA color grades round diamonds face down, through the pavilion. This is because, when viewed face up, the variation in light return significantly effects the apparent color, and makes it much more difficult to determine an accurate grade. Conversely fancy yellow diamonds are color graded face up because it's the depth of color that, far above anything else, determines the value of the diamond. Cutters of fancy yellow diamonds actually go out of their way to cut these stones with very poor light returns so they will appear more yellow. Here's an interesting snippet from a GIA article on fancy yellows. It describes how a 6+ct MRB was recut to a 4.61ct radiant in order to decrease the light return and get a higher color grade.

Cut vs Color from GIA.png

As I've previously mentioned we are looking for a 4.5ct+ OEC, but to stay within budget we need to be in the L-N color range. We have seen that some of the gorgeous OECs shown here on PS face up much whiter than their color grade, so that must mean they have excellent cuts with great light return. That's exactly what we want in our search for the "ideal" OEC. We have figured out the facet pattern we are looking for, and now I need to determine what it is that creates great light return in an OEC.

I know that for MRBs it's the crown and pavilion angles that of the utmost importance for light return. In fact about half the threads here in RT are devoted to this very subject!!! I've learned a 0.2 degree change in pavilion angle can make or break an MRB. However there's next to no discussion on this subject when it comes to OECs. When I look at the OECs on the market I see the depth percentages are all over the place, varying from below 60% to over 70%. This has to mean that the crown and pavilion angles are all over the place too. How to narrow the field and sort the wheat from the chaff without having to buy and return a zillion stones is what I've been researching the last couple of weeks. More to come, but if any of our OEC experts have already figured this out, help and advice would be much appreciated. =)2
 

prs

Brilliant_Rock
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It's a very nice diamond and faces up a heck of a lot whiter than an N. The price is right, and it would certainly get some consideration if it were a little bit bigger. I think we would prefer a little bit more of a flower petal pattern, and there seems to be a black blob under the table in some of the videos. No expert but I wonder if that's just obstruction by the camera.
 

prs

Brilliant_Rock
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I couldn't find much information when searching for the crown and pavilion angle combinations that might produce the best light return in an OEC. However there is lots of info for MRBs, including cut grade guideline charts from AGS. The guidelines have charts for MRBs varying in size from 4mm to 12mm, and table sizes varying from 47% to 70%. The following charts are for 10mm MRBs.

AGS Cut Guide 10mm MRB 57% Table.png
AGS Cut Guide 10mm MRB 53% Table.png
AGS Cut Guide 10mm MRB 50% Table.png
AGS Cut Guide 10mm MRB 47% Table.png

You can see the opportunity for ideal cut diamonds decreases with table size, but there are still plenty of combinations that will give excellent light return at the lower tables. I found these charts a little hard to read, so I simplified the 50% table chart.

AGS Cut Guide 10mm MRB 50% Table Rev A.png

The big question is how can we apply these charts to OECs? OECs generally have lower half facet lengths of 60% or less, whereas all these charts are produced for MRBs with 80% lower half facets. @Karl_K mentioned his PS Article on the effect of the pavilion mains on light return earlier in the thread, and the article also discusses the effect of the lower half facets.

I'm not expert enough to be sure I fully understand the article, but my interpretation is that lower half facets are cut about 1.0°F steeper than the pavilion main facets. So while the nominal PA is measured along the pavilion main facet, the effective PA is a combination of the nominal PA and the steeper angle of the lower half facets.

When the lower half facets are shorter, as in an OEC, the effective PA is reduced because there is considerably less pavilion area reflecting light at the steeper lower half angle. So we need to apply a correction factor to the nominal PA in order to use these charts for OECs. Let's say, for example an OEC has a CA of 37.0° and a PA of 40.8°, my thought is that when we look at this chart, we have to lower the nominal PA by this correction factor. Maybe in my example by 0.4° to 40.4°? Hopefully our light return experts can help me with the actual PA correction factor.
 

Karl_K

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40.4 would create a lot of obstruction response.
I dont know of one number that would work as a correction factor.
Everything is a combination.
What works for one set of proportions may or may not work for another.
If there was such a factor it would likely be a ratio rather than a number.
 

ForteKitty

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@prs I looked up the Sarin for my 9.9-10mm oec. 46% table, 63.5% depth, 34 deg crown, 42 deg pavillion. It has no obstruction and is bright edge to edge, and on the AGS chart for 47% table it scores a 2, so it somewhat works for oec? Sorry the image quality isn't better, the Sarin is somewhere in the house and that's an older photo.
 

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prs

Brilliant_Rock
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40.4 would create a lot of obstruction response.
I dont know of one number that would work as a correction factor.
Everything is a combination.
What works for one set of proportions may or may not work for another.
If there was such a factor it would likely be a ratio rather than a number.
Karl, are you saying that Victor Canera's CERs all create a lot of obstruction response?
 
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prs

Brilliant_Rock
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@prs I looked up the Sarin for my 9.9-10mm oec. 46% table, 63.5% depth, 34 deg crown, 42 deg pavillion. It has no obstruction and is bright edge to edge, and on the AGS chart for 47% table it scores a 2, so it somewhat works for oec? Sorry the image quality isn't better, the Sarin is somewhere in the house and that's an older photo.
Thank you so much for your response, this is very useful information indeed. I will have to spend more time tomorrow absorbing all the details. I know we can ignore the estimated GIA Facetware cut grade of "Fair". Any round diamond with a culet is doomed in their system! =)2
 

Karl_K

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Then exactly what was the point of your post?
You asked:
lower the nominal PA by this correction factor. Maybe in my example by 0.4° to 40.4

I answered.......
 

prs

Brilliant_Rock
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You asked:
lower the nominal PA by this correction factor. Maybe in my example by 0.4° to 40.4

I answered.......
Karl, as an OEC expert I assume you know that the stunningly beautiful CERs are made with a Table of 50%, a CA of 37.0%, and a PA of 40.8°. I understand Victor knows a thing or two about diamonds, so I find it both interesting and informative that, with his CA of 37.0°, he chose to go with a PA of of 40.8° instead of the matching MRB ideal PA of 40.4°. It's no coincidence those are the exact same angles I used in my example, and that's how I came up with my 0.4° correction factor.

I have found all your posts so far in this thread to be decidedly negative and discouraging. Here again without hesitation, or a moments thought, you launch into some baloney about obstruction. I welcome your participation in this thread, but wish you would use your undoubted expertise to be more helpful and informative.
 
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Karl_K

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Karl, as an OEC expert I assume you know that the stunningly beautiful CERs are made with a Table of 50%, a CA of 37.0%, and a PA of 40.8°. I understand Victor knows a thing or two about diamonds, so I find it both interesting and informative that, with his CA of 37.0°, he chose to go with a PA of of 40.8° instead of the matching MRB ideal PA of 40.4°. It's no coincidence those are the exact same angles I used in my example, and that's how I came up with my 0.4° correction factor.

I have found all your posts so far in this thread to be decidedly negative and discouraging. Here again without hesitation, or a moments thought, you launch into some baloney about obstruction. I welcome your participation in this thread, but wish you would use your undoubted expertise to be more helpful and informative.
He did not use 40.4 because it would have had obstruction issues. Which interestingly many of the real old stones have.
 

prs

Brilliant_Rock
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@prs I looked up the Sarin for my 9.9-10mm oec. 46% table, 63.5% depth, 34 deg crown, 42 deg pavillion. It has no obstruction and is bright edge to edge, and on the AGS chart for 47% table it scores a 2, so it somewhat works for oec? Sorry the image quality isn't better, the Sarin is somewhere in the house and that's an older photo.
I just found your thread on that absolutely gorgeous OEC WWW It's just amazing to finally get some cut details on such a beautiful diamond. Yes the AJS chart does give your stone a cut grade of 2, and if my correction factor works, it would be closer to a cut grade of 1.

I'm thrilled to be able to confirm these charts do have some relevance to OECs, and that figuring out an ideal cut is not as complicated as some would have us believe. It's that big blue area of very poor grades in the top right hand corner of the chart that has me concerned, and that I definitely want to avoid. I'm going to work on how to do that next.
 

prs

Brilliant_Rock
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He did not use 40.4 because it would have had obstruction issues. Which interestingly many of the real old stones have.
So why does AJS give that combination a cut grade of 1?
 

Karl_K

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So why does AJS give that combination a cut grade of 1?
In an mrb it gets a cut grade of 1 for obstruction problems(mains get dark) but the shorter lowers(bigger mains area) in an oec make it worse.
 

prs

Brilliant_Rock
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In an mrb it gets a cut grade of 1 for obstruction problems(mains get dark) but the shorter lowers(bigger mains area) in an oec make it worse.
So what pavilion angle would you recommend for an MRB with a T of 50%, a CA of 37°, and 80% lower halves?
 
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