How do LGFs affect a stone''s appearance?

[Dee*Jay]

I''ve looked at several diamonds lately and have come to the conclusion that I like longer LGFs (80-85%) more than shorter ones, but are shorter ones (e.g., around 75%) considered "better"? And how specifically do LGFs affect the look of a stone?

Recently there was a thread where John Q. posted some great pics of stones with LGFs at 75%, 80% and 85% but I''m not sure I totally grasp how the %s affect the look. I *think* longer LGFs result in narrower flashes of light (?) but that might not be correct.

And at what point would the LGF factor become noticable? I''m guessing 80%(ish) maybe? (So longer than 80% looks one way and shorter than 80% looks another?)

Finally, what %s become TOO long and TOO short?

(Sorry if these are stupid questions; I did search the threads on this before posting but I want to understand this a bit better.)

 

[belle]

Date: 7/27/2006 3:18:28 PM
Author:Dee*Jay

I *think* longer LGFs result in narrower flashes of light (?) but that might not be correct.

that''s a pretty good summary of something that is a little more complicated than what can be put in a summary.

part of the problem is, the lgf''s can be measured in 2 different ways and as much as we would like everything to be *exacting* and across the board, it''s just not.
rather than go into what i know about it (and don''t) you can read this article. i''m sure any other questions or clarification that you might need could be answered here as well.

 

[^]

Dee*Jay,

I''m still learning, but John explained to me thusly:

Narrow pavilion mains (long lower girdles) intensify & narrowly focus light return. Intense begets intense, and direct light comes back powerfully.

Wide pavilion mains (short lower girdles) = more main surface area. In soft light conditions there is more overall performance as the wide mains give back more ambient light.

There is more info in this thread

^

 

[strmrdr]

This subject could take a book....

The lgf% fine tunes the personality of a diamond the performance is determined mainly by the crown/pavilion angle relationship.

long lgf% == more pinfire and better direct light performance and somewhat less contrast which is why it works best with no painting of the girdle facets.
Painting and long lgf% will generaly make for a diamond without enough contrast.

Short lgf% == more broadfire and an increase in contrast to the point that they can look dark because the arrows are too big. Painting helps around the edges but it still makes the area under the table too dark for some people in some lighting conditions. By painting the girdles you can decrease the amound of dark areas in the diamond outside the table and it helps some: 8* does this.
Short lgf% diamonds tend to perform better in indirect and soft lighting.
Put an 8* in candle light and it will blow away most of other round cuts.
But it isn''t free it also effects other aspects of a diamonds performance most noticeable in the scintialltion of the diamond.

I prefer long lgf% diamonds with traditional girdles but some like em the other way.

Take a bad combo of crown and pavilion angles and the LGF is pretty irrelevant it cant make a silk purse out a sows ear.
On the other hand the lgf% can work in conjunction with other aspects of the cut to improve or in some cases degrade performance but mostly it determines the personality of the diamond.

 

[JohnQuixote]

Appreciate the links and input here.

It’s important to point out that overall configuration, including optical symmetry, has much to do with how the minors – lower girdles in particular – influence the character of the diamond. It all depends on the total formula.

Different people have different ‘sweet spots,’ and all will depend on overall configuration. It’s possible to have beautiful overall performance with LG Halves on the short or long side.

Date: 7/27/2006 4:38:47 PM
Author: ^

I'm still learning, but John explained to me thusly:

Narrow pavilion mains (long lower girdles) intensify & narrowly focus light return. Intense begets intense, and direct light comes back powerfully.

Wide pavilion mains (short lower girdles) = more main surface area. In soft light conditions there is more overall performance as the wide mains give back more ambient light.

That's the sumup I would have used for Dee*Jay, as well as the below...

Date: 7/27/2006 4:40:52 PM
Author: strmrdr

The lgf% fine tunes the personality of a diamond the performance is determined mainly by the crown/pavilion angle relationship.

long lgf% == more pinfire and better direct light performance and somewhat less contrast.

Short lgf% == more broadfire and an increase in contrast.

Strm, before committing to painting statements I would want to isolate whether we're talking about crown-only painting and to what degree - or pavilion painting and to what degree - or both and to what degrees. In many instances there would not be any visible influence. There might be if we're only talking about diamonds a few degrees outside of GIA EX.

 

[Dee*Jay]

Thank you all very much for the info!

The thread that you linked, Storm, was what really got me thinking about the impact LGFs have on performance.

John Q., as always a wealth of info!

And Belle, I know I know -- A *small* question that would require a *big* answer! I was afraid it wasn't as simple as I was making it out to be.

So... how long/short can you go without impacting performance in a negative way? For instance, I just got a stone with 85% LGFs (I know this is a rounded number and could be a couple of % points in either direction). Can you go to say 90? Or on the short end, does 70 make it an unattractive stone? (The answer to all of this, of course, might be totally subjective; a diamond that one person likes would not be one that antoher would prefer [to a point of course]).

ETA: Didn't say or ask anything about painting becuase honestly the painting discussion often take off to a place so far over my head that I won't even pretend to begin to understand them.

 

[JohnQuixote]

Date: 7/27/2006 5:22:37 PM
Author: Dee*Jay
Thank you all very much for the info!

The thread that you linked, Storm, was what really got me thinking about the impact LGFs have on performance.

John Q., as always a wealth of info!

And Belle, I know I know -- A *small* question that would require a *big* answer! I was afraid it wasn't as simple as I was making it out to be.

So... how long/short can you go without impacting performance in a negative way? For instance, I just got a stone with 85% LGFs (I know this is a rounded number and could be a couple of % points in either direction). Can you go to say 90? Or on the short end, does 70 make it an unattractive stone? (The answer to all of this, of course, might be totally subjective; a diamond that one person likes would not be one that antoher would prefer [to a point of course]).

ETA: Didn't say or ask anything about painting becuase honestly the painting discussion often take off to a place so far over my head that I won't even pretend to begin to understand them.

LOL Dee*Jay. We do like to get into details around here don't we?

Ok short answer If the main angles are good and it has decent or better optical symmetry you should see it sizzle and pop nicely in direct light. 90 and 70 would be pretty extreme. In my experience the practical range is inside those numbers the way GIA measures but, again, you can't say which tie looks best with the suit until you know the colors and style of the suit.

If you have an ideal-scope or magnified photo and the main measurements I can give you an estimate of the actual %.

 

[strmrdr]

Date: 7/27/2006 5:22:37 PM
Author: Dee*Jay
Thank you all very much for the info!


The thread that you linked, Storm, was what really got me thinking about the impact LGFs have on performance.


John Q., as always a wealth of info!


And Belle, I know I know -- A *small* question that would require a *big* answer! I was afraid it wasn''t as simple as I was making it out to be.


So... how long/short can you go without impacting performance in a negative way? For instance, I just got a stone with 85% LGFs (I know this is a rounded number and could be a couple of % points in either direction). Can you go to say 90? Or on the short end, does 70 make it an unattractive stone? (The answer to all of this, of course, might be totally subjective; a diamond that one person likes would not be one that antoher would prefer [to a point of course]).


ETA: Didn''t say or ask anything about painting becuase honestly the painting discussion often take off to a place so far over my head that I won''t even pretend to begin to understand them.

In general 77-81 is a sweet spot for overall all around performance
80+ for people like me who love bright diamonds over fiery diamonds but id take 78-79 too.
For the right person 85% is kicken but someone else might not like it.
Same with 75% or even 70% on the other end.
It comes down to a matter of what you prefere.
Id likely love a 85% lgf diamond :}
I think its kicken that you have one and love it :}

 

[strmrdr]

te:[/b] 7/27/2006 5:13:23 PM
Author: JohnQuixote



Strm, before committing to painting statements I would want to isolate whether we''re talking about crown-only painting and to what degree - or pavilion painting and to what degree - or both and to what degrees. In many instances there would not be any visible influence. There might be if we''re only talking about diamonds a few degrees outside of GIA EX.

[/quote]

Painting as seen in 8* and new line ACA where the contrast leakage is eliminated but not extreme painting.
Right now I cant think of a way other than that to describe it that wouldnt get too confusing and open a can of worms.
As you know there are several ways to achieve that effect.
And yea I said painting can be a good thing with some trade offs
LOL

 

[Dee*Jay]

John - I just got the following stone:

9.65 X 9.70 X 5.91
61.1% depth
56% table
35 crown angle
15.5% crown height
40.4 pavilion angle
42.5% pavilion depth
55% star length
85% lower half
girdle thin to medium, faceted
culet none

It only scores a .6 on the HCA but I don't *think* I'm having a lack of contrast problem. (Or maybe I'm just too blind/stupid to know it, which is perfectly possible too!) I also tried to run it on the free version of Diamond Calc last night but it didn't work. I sure hope I'm not having a ton of leakage, especially under the table. Yikes!

ETA - here's a link to some photos, but I don't know if this will be useful to you at all (probably not... ).

And Storm, the 85% (+/-) length is turning out to be one of my problems. I'm looking at other stones and I find that I like the longer LGFs, which are not a common as you would think, especially when you throw in the color, clarity and relevant angles that I want too. Jeesh, as if all the other things weren't enough, now the LGFs... !

So, in a nutshell I'm trying to learn about LGFs to see if it will become one of my deciding factors in a potential upgrade stone. (There is also the *possiblity* that I *might* just keep this stone if it turns out to be OK--but don't tell my husband that that's a potential course of action because I want to keep him in upgrade mode until I for sure decide!!!)

 

[JohnQuixote]

Date: 7/27/2006 5:47:03 PM
Author: strmrdr

Painting as seen in 8* and new line ACA where the contrast leakage is eliminated but not extreme painting.
Right now I cant think of a way other than that to describe it that wouldnt get too confusing and open a can of worms.
As you know there are several ways to achieve that effect.
And yea I said painting can be a good thing with some trade offs
LOL

I can help you narrow it down. With ACA NL it would be crown-only painting.
Wink or Bill may be able to comment on 8*. I suspect it's crown-only as well (?)

Then we need to know if we're talking about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11 degrees of painting?

Contrast leakage may be eliminated with 2-4 degrees of painting if the diamond is well cut, but some character traits don't begin until several degrees after that. If you've seen a diamond with 7 or 8 degrees average upper-half deviation the look is different than one with 3 or 4 degrees.

 

[JohnQuixote]

Date: 7/27/2006 6:19:16 PM
Author: Dee*Jay
John - I just got the following stone:

9.65 X 9.70 X 5.91
61.1% depth
56% table
35 crown angle
15.5% crown height
40.4 pavilion angle
42.5% pavilion depth
55% star length
85% lower half
girdle thin to medium, faceted
culet none

It only scores a .6 on the HCA but I don''t *think* I''m having a lack of contrast problem. (Or maybe I''m just too blind/stupid to know it, which is perfectly possible too!) I also tried to run it on the free version of Diamond Calc last night but it didn''t work. I sure hope I''m not having a ton of leakage, especially under the table. Yikes!

ETA - here''s a link to some photos, but I don''t know if this will be useful to you at all (probably not... ).

And Storm, the 85% (+/-) length is turning out to be one of my problems. I''m looking at other stones and I find that I like the longer LGFs, which are not a common as you would think, especially when you throw in the color, clarity and relevant angles that I want too. Jeesh, as if all the other things weren''t enough, now the LGFs... !

So, in a nutshell I''m trying to learn about LGFs to see if it will become one of my deciding factors in a potential upgrade stone. (There is also the *possiblity* that I *might* just keep this stone if it turns out to be OK--but don''t tell my husband that that''s a potential course of action because I want to keep him in upgrade mode until I for sure decide!!!)

Dee*Jay - I couldn''t find the link?

It looks like a GIA grading report, correct? The main numbers are all rounded, but we have the ct weight and mm measurements. If you had an ideal-scope or mag photo face up one of us could import the photo into DiamCalc and maybe tell you more precisely what the actual numbers are. Of course, a Sarin or Helium scan would give it to you more reliably.

In any case those numbers won''t show leakage and a bunch of us fiddling around with DiamCalc can''t improve upon the best measuring tool available - your eyes.

 

[Dee*Jay]

Oh I'm such a dingbat. Here's the thread: link

There are a couple of photos as you scroll down that show the arrows. Sorry I don't have enough useful info (no helium scan or anything like that... ), but thank you for offering to look at it anyway!

So... is it feasible/reasonable to add the LGF into the mix when looking for a new stone or should I just thank my lucky stars if I found one between 1 and 2 on the HCA and not worry about the LGFs?

 

[Garry H (Cut Nut)]

I like steep crown shallow pavilion firey ideal''s with small tables and very long lower girdle facets.

They do not have as much diameter spread, but they point out of a setting like Madonna''s famous bra.
And in +1ct or bigger stoes they have wonderful scintillation.

So LG optimum facet length also depends on table size, and crown pavilion angles.

 

[Dee*Jay]

Garry - so with this info do I keep it or keep looking?

61.1% depth
56% table
35 crown angle
15.5% crown height
40.4 pavilion angle
42.5% pavilion depth
55% star length
85% lower half
girdle thin to medium, faceted
culet none
HCA = .6
3.37 ct

I *like* this stone but I'm willing to move on if I need to.

 

[JohnQuixote]

The diamond sounds like a great mix of elements Dee*Jay. Garry was voicing his approval.

Diamonds are rare in that size and it sounds like you have a winner. I enjoyed the link.

 

[Dee*Jay]

Thanks John. I *thought* Garry was giving it the thumbs up but just wanted to make sure he wasn''t commenting on that kind of diamond in general.

By the way, there isn''t much else I own that will ever be likened to Madonna''s bra. Hmmmm... I''m getting some interesting setting ideas...

 

[strmrdr]

Date: 7/27/2006 9:21:16 PM
Author: Dee*Jay
Garry - so with this info do I keep it or keep looking?

61.1% depth
56% table
35 crown angle
15.5% crown height
40.4 pavilion angle
42.5% pavilion depth
55% star length
85% lower half
girdle thin to medium, faceted
culet none
HCA = .6
3.37 ct

I *like* this stone but I'm willing to move on if I need to.

uncommon but I like it... looks like a stunning diamond.

 

[strmrdr]

btw with those angles and table a short lgf% would be one of the cases where it can hurt performance.
imho whoever cut it knew what they were doing when they chose the combo.

 

[Garry H (Cut Nut)]

85% is a bit long - but GIA rounding is so bad that it could be exceptional - we need to see a ideal-scope photo to be sure.

It will be a great stone though - and the longish lg''s = more pin fire. Not bad - just "not usual" if they really are 85%. It would look like a tolkowsky with 83%
but if they were 75% it would be bad.

 

[Dee*Jay]

Well that settles it -- I''ll keep the stone and change the setting. This turned out to be easier than I thought. (Ha! Famous last words!)

Thank you guys!