Low LGF%? Need advice on this stone

[echelon6]

Hi all. Hoping some experts can chime in with regards to any vices that may be evident in this stone:

1.588ct
Diameter: 7.46-7.48mm
Depth: 61.8% (too deep?)
Table: 56.2%
Crown angle: 34.8 (34.6 - 34.9)
Pavilion angle: 40.8 (40.7 - 40.9)
Crown height: 15.3%
Pavilion depth: 43.1%
Star/Upper ratio: 51:49
Lower Girdle Halves: 74%

Very worried about the Lower Girdle Halves ratio of 74%, I don''t want a dark diamond, but I like fire more than white light. Can I pls get some expert opinions on this?

Also I read (but didn''t understand fully) that there are certain combinations of Star% vs LGF%s that make the stone just fine, so does the above stone have a good combination?

Thanks

 

[strmrdr]

74 is below my tolerance level but yours may be different.
61.8 isnt too deep.

 

[WinkHPD]

LOL, another area for Storm and I to disagree, I love the larger flashes of light and color caused by the shorter LGF''s. Storm likes the narrower. This is truly a personal preference issue.

By the way, if any one has any questions about Storm and I, we are friends who frequently disagree, but who respect each other while doing so.

Wink

 

[Cehrabehra]

Date: 7/14/2007 2:20:06 PM
Author: strmrdr
74 is below my tolerance level but yours may be different.
61.8 isnt too deep.

Storm likes longer lgf - its a personal preference thing. I used to like only the short ones like under 75 but the longer I''m around here and diamonds in general the more I like pretty much every combination LOL Its just good to know that they''re differen''t appearances/personalities/flavors. The longer lgf give longer splinters of light and shorter is a bit chunkier. DeeJay has very long lgf in her stone and its really beautiful.

 

[echelon6]

Date: 7/14/2007 2:20:06 PM
Author: strmrdr
74 is below my tolerance level but yours may be different.

61.8 isnt too deep.

I''ve read all your prev posts and replies on LGF%, and it seems you''re an expert in this field. Can you please comment more on why 75% is too low for this particular stone?

You also mentioned a couple of times before how higher Pav angles (e.g. 40.8 - 40.9) mean that its OK to have a low LGF%. Why is that? Is it the case for this stone?

Also the combination of Star 51% / LGF 74%, is that problematic in any way?

I don''t mind chunkier fire per se, but if the low LGF makes for a dark diamond, or makes it look yellow (color entrapment?) then I''d pass on this stone.

Also, I never bought into all that talk about contrast, I see it as a bad thing since it is achieved through having leakage. If a low LGF increases contrast at the sacrifice of light return, I''d pass this stone.

Very interested in your opinions on this.

Thanks

 

[strmrdr]

I want to make it very clear that I am a consumer not one of the board experts.
Lets see if I can answer some of your questions without getting in another fight.

1: lgf% and pavilion angle: its not so much the pavilion angle as the crown/pavilion combo.
some combos will show the effects of the shorter lgf% more.
Which is why one of the key properties of an 8* is the angle combo.
Their selling point is in your face as much as possible arrows hence 8*

2: The color of clothes someone typically wears can make a difference,
dark clothes will make the effect of a short lgf% more so.

3: its a lot about personal preference.

4: color entrapment is a non-issue.

5: most contrast is a well cut RB is caused by obstruction(reflecting the surroundings)
leakage is secondary but important.
Contrast brilliance is a critical part of a stones performance both static(brilliance) and moving(scint.).

6: too much obstruction(short lgf%) in some lighting to me looks too darkish.
some disagree, others will say that 74 isn't short.

7: the bad leakage is under the table, leakage around the perimeter of the stone in the right places aids contrast brilliance and scint.

 

[echelon6]

Date: 7/14/2007 9:45:21 PM
Author: strmrdr
I want to make it very clear that I am a consumer not one of the board experts.

Lets see if I can answer some of your questions without getting in another fight.


1: lgf% and pavilion angle: its not so much the pavilion angle as the crown/pavilion combo.

some combos will show the effects of the shorter lgf% more.

Which is why one of the key properties of an 8* is the angle combo.

Their selling point is in your face as much as possible arrows hence 8*


2: The color of clothes someone typically wears can make a difference,

dark clothes will make the effect of a short lgf% more so.

Right, so this doesnt happen with longer LGF''s, correct? Therefore this is to be construed as a disadvantage of lower LGFs, correct?

3: its a lot about personal preference.

The reasonable person (myself included) would prefer a diamond with more light return than a dark one. If a low LGF causes a dark diamond, theres no contest there.

4: color entrapment is a non-issue.


5: most contrast is a well cut RB is caused by obstruction(reflecting the surroundings)

leakage is secondary but important.

Contrast brilliance is a critical part of a stones performance both static(brilliance) and moving(scint.).


6: too much obstruction(short lgf%) in some lighting to me looks too darkish.

some disagree, others will say that 74 isn''t short.

So we''ve established that a short LGF makes a dull diamond, therefore all should go for an LGF range of 78-82?

7: the bad leakage is under the table, leakage around the perimeter of the stone in the right places aids contrast brilliance and scint.

I apologise if I''m being too black and white here in my interpretations and inferences, but it really does help me reach a decision.
Thanks

 

[strmrdr]

its not that absolute...

dark clothing will make the arrows of a long lgf% diamond stand out more/darken also and I should have mentioned skin tone, hair color and hair size too.
But with a short lgf% its a larger part of the diamond, so if it already has too much obstruction for someone dark clothing/skin tone/hair can make it worse.

"The reasonable person (myself included) would prefer a diamond with more light return than a dark one. If a low LGF causes a dark diamond, there''s no contest there."

Like anything else its a trade off, chunky fire vs obstruction.
Love chunky fire might not mind the trade off.

"So we''ve established that a short LGF makes a dull diamond, therefore all should go for an LGF range of 78-82?"
dull isn''t the right word because it can be fiery, lets say less white light return and different fire.
If I was the diamond king for a day Id set the min at 77% but Wink would cry.

 

[junior35]

"Traditionally" ,before the discovery of hearts and arrows ,the combination of 80/55 was widely accepted.That is 80% lgf and 55% star facets.
Later,it was noticed that shorter (e.g. 75%) lgf produce fatter arrows.So now we get to the sensitive area of taste.I personally like slightly fatter arrows (77%)(see attached pic) .But it seems that most of the industry likes 80%+(e.g.) thinner arrows .It''s a matter of taste.There is no right or wrong.

 

[WorkingHardforSmallRewards]

wow, that is really beautiful. Got to love those colors at least.

 

[echelon6]

Appreciate the comments all. Many thanks

So I conclude:

Low LGF = chunkier fire, at the cost of lower light return since the main facets are blocked most of the time by the viewer''s head etc?

Then why did I read previously that low LGF stones perform better in indirect, weaker light? Wouldnt the opposite be true since the arrow areas are larger? Since low LGF stones are more dependent on direct light, thats what I infer...

Also could I get a few more expert opinions on the bare fact of a LGF of 74%? Will that make for a darker diamond overall, compared to a 78-82 LGF?

 

[Cehrabehra]

Date: 7/14/2007 10:18:53 PM
Author: strmrdr
its not that absolute...

dark clothing will make the arrows of a long lgf% diamond stand out more/darken also and I should have mentioned skin tone, hair color and hair size too.
But with a short lgf% its a larger part of the diamond, so if it already has too much obstruction for someone dark clothing/skin tone/hair can make it worse.

''The reasonable person (myself included) would prefer a diamond with more light return than a dark one. If a low LGF causes a dark diamond, there''s no contest there.''

Like anything else its a trade off, chunky fire vs obstruction.
Love chunky fire might not mind the trade off.

''So we''ve established that a short LGF makes a dull diamond, therefore all should go for an LGF range of 78-82?''
dull isn''t the right word because it can be fiery, lets say less white light return and different fire.
If I was the diamond king for a day Id set the min at 77% but Wink would cry.

Storm is right here..... and it is probably one of the reasons I love my stone so much more than others who have seen similar stones - I am very fair, blonde, smallish head, not big hair, the colors that reflect to me are apricots and pinks and my natural colors.... except when I wear black. I could see storm and his big head (haha!) liking the longer lgf while to me the chunkier facets are throwing back pastel colors, not black and dark.

 

[Cehrabehra]

Date: 7/15/2007 12:18:37 AM
Author: echelon6
Appreciate the comments all. Many thanks

So I conclude:

Low LGF = chunkier fire, at the cost of lower light return since the main facets are blocked most of the time by the viewer''s head etc?

Then why did I read previously that low LGF stones perform better in indirect, weaker light? Wouldnt the opposite be true since the arrow areas are larger? Since low LGF stones are more dependent on direct light, thats what I infer...

Also could I get a few more expert opinions on the bare fact of a LGF of 74%? Will that make for a darker diamond overall, compared to a 78-82 LGF?

IMO the "blocked" thing is only partly true.... those facets will STILL reflect what they see.... if you''re wearing a bright red shirt, guess what they will see? And if they see red, guess what you will see? If you are dark, they will see dark and so will you. Yes, it blocks some of the direct and indirect light, but IME that''s when the environmental colors are most obvious.

 

[strmrdr]

Date: 7/15/2007 12:18:37 AM
Author: echelon6
Appreciate the comments all. Many thanks

Then why did I read previously that low LGF stones perform better in indirect, weaker light? Wouldnt the opposite be true since the arrow areas are larger? Since low LGF stones are more dependent on direct light, thats what I infer...

overly simple answer at 1am....
low lgf% stones perform well in low light because they return bigger chunks of light which is easier to see.

 

[Garry H (Cut Nut)]

Date: 7/14/2007 11:25:06 PM
Author: junior35
''Traditionally'' ,before the discovery of hearts and arrows ,the combination of 80/55 was widely accepted.That is 80% lgf and 55% star facets.
Later,it was noticed that shorter (e.g. 75%) lgf produce fatter arrows.So now we get to the sensitive area of taste.I personally like slightly fatter arrows (77%)(see attached pic) .But it seems that most of the industry likes 80%+(e.g.) thinner arrows .It''s a matter of taste.There is no right or wrong.

Wow Junior, that is one seriously painted stone!!!!!!!!!!
And the star facets are unusually short too?

I think a taste for this particular stone would be very personal. It would no doubt be very firey, but it would not have lots of brilliance?

 

[echelon6]

Thanks for all the input

I think I will pass on this stone. Problem is, all the other ACA's dont have much higher LGF%s either. (see my ACA thread)

A real shame cos this stone is the culmination of a week's of searching, and in my opinion, is the tightest, most symmetrical ACA I found in WF's inventory. Also turns out it has the lowest LGF...

 

[strmrdr]

Honestly at this point you have several 1/2 to 3/4 understood concepts in your head.
If your not in a hurry kick back for a week and study the tutorials here at WF and GOG and read some threads then get back into the hunt.
Taking a day off and relaxing isn't a bad idea either.

 

[Paul-Antwerp]

Hohoho. Can we have a small reality check here?

I see way too many incorrect statements about LGF%. May I try to correct these?

1. LGF-percentage as such has no meaning

If a young tennis-player wins his first ever match, he has a 100% winning result. Is he a better player than the professional player who won 80% of all his matches? In the same way LGF-% has no meaning. Take a look back and consider what the LGF-% really is.

The LGF-% is a result of the relationship between the lower-girdle-facets (LGF) and the main-pavilion-facets (MPF). The smaller the difference in angle between LGF and MPF, the longer the LGF and thus the LGF-%. The bigger the difference, the shorter the LGF-%.

If we look at the pavilion only, with it being the foundation of light return, the surface of the LGF is a lot higher than that of the MPF. One could therefore conclude that the angle of the LGF is probably more important than the angle of the MPF. Unfortunately, it is easier to measure the angle of the MPF, and because this has become an historical practice, it might lead to a lot of incorrect conclusions.

So, if we have an angle of 41.5° on the LGF, the LGF-% will be longer if we have an MPF-angle of 41° as opposed to having an MPF-angle of 40.6°. The basic light return of the LGF''s however will be the same, with the only exception that the surface will be smaller in the latter case.

2. Lenght of LGF has an effect on scintillation

With longer LGF''s, the virtual facets become smaller, which will increase the possibility of small-size-scintillation. In the same way, shorter LGF''s increases virtual facets, thus increases the likelihood of broad flash fire. This is a taste-aspect, but also needs to be looked at in relation to the size of the stone. In very small stones, it is rather dangerous to decrease virtual-facet-size, since sparkle might become inobservable.

3. Short and long LGF''s are terms that have lost their meaning

Not so long ago, I thought that short LGF''s refered to percentages around 65 to max 70% and long LGF''s to anything above 80%. Now, I see 74% described as short, which is ludicrous. I know, in basketball, 6 feet is rather short, but not in normal life. In the same way, 74% is not short. Add to this the difficulty of exactly measuring this percentage.

I hope that this brings us back to earth.

Live long,

 

[junior35]

Date: 7/15/2007 2:15:02 AM
Author: Garry H (Cut Nut)

Date: 7/14/2007 11:25:06 PM
Author: junior35
''Traditionally'' ,before the discovery of hearts and arrows ,the combination of 80/55 was widely accepted.That is 80% lgf and 55% star facets.
Later,it was noticed that shorter (e.g. 75%) lgf produce fatter arrows.So now we get to the sensitive area of taste.I personally like slightly fatter arrows (77%)(see attached pic) .But it seems that most of the industry likes 80%+(e.g.) thinner arrows .It''s a matter of taste.There is no right or wrong.

Wow Junior, that is one seriously painted stone!!!!!!!!!!
And the star facets are unusually short too?

I think a taste for this particular stone would be very personal. It would no doubt be very firey, but it would not have lots of brilliance?

Gary,this is a semi-finished stone from a Dialit automatic machine. It''s fully brillianteered on the bottom and 8/8 on the top.

 

[diagem]

Date: 7/15/2007 7:59:13 AM
Author: Paul-Antwerp
Hohoho. Can we have a small reality check here?

I see way too many incorrect statements about LGF%. May I try to correct these? I agree!

1. LGF-percentage as such has no meaning

If a young tennis-player wins his first ever match, he has a 100% winning result. Is he a better player than the professional player who won 80% of all his matches? In the same way LGF-% has no meaning. Take a look back and consider what the LGF-% really is.

The LGF-% is a result of the relationship between the lower-girdle-facets (LGF) and the main-pavilion-facets (MPF). The smaller the difference in angle between LGF and MPF, the longer the LGF and thus the LGF-%. The bigger the difference, the shorter the LGF-%.

I agree that the smaller the difference in angle between the LGF and MPF is naturaly on the longer LGF...
But once you go with the shorter LGF''s you have more options and still keep the difference between angle to a minimum (painting!!!)
Am I making sense?

If we look at the pavilion only, with it being the foundation of light return, the surface of the LGF is a lot higher than that of the MPF. One could therefore conclude that the angle of the LGF is probably more important than the angle of the MPF. Unfortunately, it is easier to measure the angle of the MPF, and because this has become an historical practice, it might lead to a lot of incorrect conclusions.

Makes complete sense, and can change the way we conclude things...

So, if we have an angle of 41.5° on the LGF, the LGF-% will be longer if we have an MPF-angle of 41° as opposed to having an MPF-angle of 40.6°. The basic light return of the LGF''s however will be the same, with the only exception that the surface will be smaller in the latter case.

2. Lenght of LGF has an effect on scintillation

With longer LGF''s, the virtual facets become smaller, which will increase the possibility of small-size-scintillation. In the same way, shorter LGF''s increases virtual facets, thus increases the likelihood of broad flash fire. This is a taste-aspect, but also needs to be looked at in relation to the size of the stone. In very small stones, it is rather dangerous to decrease virtual-facet-size, since sparkle might become inobservable.

3. Short and long LGF''s are terms that have lost their meaning

Not so long ago, I thought that short LGF''s refered to percentages around 65 to max 70% and long LGF''s to anything above 80%. Now, I see 74% described as short, which is ludicrous. I know, in basketball, 6 feet is rather short, but not in normal life. In the same way, 74% is not short. Add to this the difficulty of exactly measuring this percentage.

74% is in my opinion long..., i agree.

I hope that this brings us back to earth.

Live long,

 

[strmrdr]

Date: 7/15/2007 7:59:13 AM
Author: Paul-Antwerp

3. Short and long LGF''s are terms that have lost their meaning

Not so long ago, I thought that short LGF''s refered to percentages around 65 to max 70% and long LGF''s to anything above 80%. Now, I see 74% described as short, which is ludicrous. I know, in basketball, 6 feet is rather short, but not in normal life. In the same way, 74% is not short. Add to this the difficulty of exactly measuring this percentage.

I hope that this brings us back to earth.

Live long,

Historically speaking 35.5/41.5 isn''t steep deep but that is what we call it.
62% is a med table historically too.
Times change.

 

[echelon6]

Date: 7/15/2007 7:59:13 AM
Author: Paul-Antwerp
Hohoho. Can we have a small reality check here?


I see way too many incorrect statements about LGF%. May I try to correct these?


1. LGF-percentage as such has no meaning


If a young tennis-player wins his first ever match, he has a 100% winning result. Is he a better player than the professional player who won 80% of all his matches? In the same way LGF-% has no meaning. Take a look back and consider what the LGF-% really is.


The LGF-% is a result of the relationship between the lower-girdle-facets (LGF) and the main-pavilion-facets (MPF). The smaller the difference in angle between LGF and MPF, the longer the LGF and thus the LGF-%. The bigger the difference, the shorter the LGF-%.


If we look at the pavilion only, with it being the foundation of light return, the surface of the LGF is a lot higher than that of the MPF. One could therefore conclude that the angle of the LGF is probably more important than the angle of the MPF. Unfortunately, it is easier to measure the angle of the MPF, and because this has become an historical practice, it might lead to a lot of incorrect conclusions.


So, if we have an angle of 41.5° on the LGF, the LGF-% will be longer if we have an MPF-angle of 41° as opposed to having an MPF-angle of 40.6°. The basic light return of the LGF''s however will be the same, with the only exception that the surface will be smaller in the latter case.


2. Lenght of LGF has an effect on scintillation



With longer LGF''s, the virtual facets become smaller, which will increase the possibility of small-size-scintillation. In the same way, shorter LGF''s increases virtual facets, thus increases the likelihood of broad flash fire. This is a taste-aspect, but also needs to be looked at in relation to the size of the stone. In very small stones, it is rather dangerous to decrease virtual-facet-size, since sparkle might become inobservable.


3. Short and long LGF''s are terms that have lost their meaning



Not so long ago, I thought that short LGF''s refered to percentages around 65 to max 70% and long LGF''s to anything above 80%. Now, I see 74% described as short, which is ludicrous. I know, in basketball, 6 feet is rather short, but not in normal life. In the same way, 74% is not short. Add to this the difficulty of exactly measuring this percentage.


I hope that this brings us back to earth.


Live long,

Thanks for the reply, Paul. Very informative and I did learn a few things.

So basically LGF length is determined by the proximity of LGF and MPF angles - the closer they are, the larger the LGF.

But what I still don''t understand is why there''s a consensus that LGF should fall within 77-81%. There must be a reason, i.e. this range is optimum for overall well-rounded performance of a diamond, and any LGF outside this range should be cause for concern?

And I mean if it was 76%, I''d just accept it. But at 74%, that must be like 1 or 2 standard deviations away from the mean, amongst superideals. I certainly get that impression from my many hours of browsing vendor inventory...

 

[junior35]

Date: 7/15/2007 11:03:20 AM
Author: strmrdr

Date: 7/15/2007 7:59:13 AM
Author: Paul-Antwerp

3. Short and long LGF''s are terms that have lost their meaning

Not so long ago, I thought that short LGF''s refered to percentages around 65 to max 70% and long LGF''s to anything above 80%. Now, I see 74% described as short, which is ludicrous. I know, in basketball, 6 feet is rather short, but not in normal life. In the same way, 74% is not short. Add to this the difficulty of exactly measuring this percentage.

I hope that this brings us back to earth.

Live long,

Historically speaking 35.5/41.5 isn''t steep deep but that is what we call it.
62% is a med table historically too.
Times change.

strmrdr,I second that.Tastes change and what''s more,all these discussions here really target north american tastes.
There is a huge market in Asia (ex Japan) that rejects smallish tables found in ideal cuts and prefers larger
tables (60%+).

 

[WinkHPD]

Date: 7/15/2007 11:12:21 AM
Author: echelon6

Date: 7/15/2007 7:59:13 AM

Author: Paul-Antwerp

Hohoho. Can we have a small reality check here?



I see way too many incorrect statements about LGF%. May I try to correct these?



1. LGF-percentage as such has no meaning



If a young tennis-player wins his first ever match, he has a 100% winning result. Is he a better player than the professional player who won 80% of all his matches? In the same way LGF-% has no meaning. Take a look back and consider what the LGF-% really is.



The LGF-% is a result of the relationship between the lower-girdle-facets (LGF) and the main-pavilion-facets (MPF). The smaller the difference in angle between LGF and MPF, the longer the LGF and thus the LGF-%. The bigger the difference, the shorter the LGF-%.



If we look at the pavilion only, with it being the foundation of light return, the surface of the LGF is a lot higher than that of the MPF. One could therefore conclude that the angle of the LGF is probably more important than the angle of the MPF. Unfortunately, it is easier to measure the angle of the MPF, and because this has become an historical practice, it might lead to a lot of incorrect conclusions.



So, if we have an angle of 41.5° on the LGF, the LGF-% will be longer if we have an MPF-angle of 41° as opposed to having an MPF-angle of 40.6°. The basic light return of the LGF''s however will be the same, with the only exception that the surface will be smaller in the latter case.



2. Lenght of LGF has an effect on scintillation





With longer LGF''s, the virtual facets become smaller, which will increase the possibility of small-size-scintillation. In the same way, shorter LGF''s increases virtual facets, thus increases the likelihood of broad flash fire. This is a taste-aspect, but also needs to be looked at in relation to the size of the stone. In very small stones, it is rather dangerous to decrease virtual-facet-size, since sparkle might become inobservable.



3. Short and long LGF''s are terms that have lost their meaning





Not so long ago, I thought that short LGF''s refered to percentages around 65 to max 70% and long LGF''s to anything above 80%. Now, I see 74% described as short, which is ludicrous. I know, in basketball, 6 feet is rather short, but not in normal life. In the same way, 74% is not short. Add to this the difficulty of exactly measuring this percentage.



I hope that this brings us back to earth.



Live long,

Thanks for the reply, Paul. Very informative and I did learn a few things.


So basically LGF length is determined by the proximity of LGF and MPF angles - the closer they are, the larger the LGF.


But what I still don''t understand is why there''s a consensus that LGF should fall within 77-81%. There must be a reason, i.e. this range is optimum for overall well-rounded performance of a diamond, and any LGF outside this range should be cause for concern?


And I mean if it was 76%, I''d just accept it. But at 74%, that must be like 1 or 2 standard deviations away from the mean, amongst superideals. I certainly get that impression from my many hours of browsing vendor inventory...

Consensus? Where is this consensus? I have not heard or seen it.

Wink

 

[Paul-Antwerp]

Date: 7/15/2007 11:12:21 AM
Author: echelon6

Thanks for the reply, Paul. Very informative and I did learn a few things.

So basically LGF length is determined by the proximity of LGF and MPF angles - the closer they are, the larger the LGF.

But what I still don''t understand is why there''s a consensus that LGF should fall within 77-81%. There must be a reason, i.e. this range is optimum for overall well-rounded performance of a diamond, and any LGF outside this range should be cause for concern?

And I mean if it was 76%, I''d just accept it. But at 74%, that must be like 1 or 2 standard deviations away from the mean, amongst superideals. I certainly get that impression from my many hours of browsing vendor inventory...

If you consider that nowadays many cutters cut the LGF first and the main pavilion facets afterwards, it might lead you to think differently again.

I do not understand where you find that consensus. I do not see such consensus, and if you see it, it is probably due to a too limited selection.

Finally, the same stone measured on one machine compared to another could well give a different result by 2 to 3%, with the difference becoming higher as the LGF-% increases.

''Ground control to major Tom''

 

[diamondseeker2006]

You know what I think? I think you are overanalyzing. I have one diamond with 80% lgf and two diamonds that have 75%. Honest to goodness, I just had to look up the numbers because off hand I could not have told you which was which. Yes, it may be a personal preference thing, but you have to realize if a stone is graded AGS0, then it has great light return. I have been guilty of overanalyzing the numbers in the past as well, so I understand. But I just think rejecting an ACA because it has an lgf of 74 is kind of funny, seeing that I doubt seriously you could tell the difference between 74 and 75 with your eyes.

 

[echelon6]

Date: 7/15/2007 12:09:07 PM
Author: Paul-Antwerp


If you consider that nowadays many cutters cut the LGF first and the main pavilion facets afterwards, it might lead you to think differently again.


I do not understand where you find that consensus. I do not see such consensus, and if you see it, it is probably due to a too limited selection.


Finally, the same stone measured on one machine compared to another could well give a different result by 2 to 3%, with the difference becoming higher as the LGF-% increases.


''Ground control to major Tom''

I read in your reply to my other post that LGF can not be interpreted separate from main pav angles. So for this particular stone, with pav angles of 40.8 (40.7-40.9), does that somehow excuse a LGF of 74%?

 

[echelon6]

Also, the consensus of ideal LGF range I refer to can be found here: https://www.whiteflash.com/diamonds_info/t/all_about.aspx?articleid=299&zoneid=21
(In the criteria for ACA, it says LGF = 76-80%)

That''s according to WF. But it''s also been supported in numerous posts by Rhino, John Q, Garry and a few others (sorry, can''t find the links right now)

 

[Paul-Antwerp]

No need for such a stone to be excused. The difference is minimal, dependant upon the correctness of the measurement and in the end, it is a taste-issue, not a performance-issue.

Live long,

 

[Paul-Antwerp]

Date: 7/15/2007 12:34:48 PM
Author: echelon6

Date: 7/15/2007 12:09:07 PM
Author: Paul-Antwerp


If you consider that nowadays many cutters cut the LGF first and the main pavilion facets afterwards, it might lead you to think differently again.


I do not understand where you find that consensus. I do not see such consensus, and if you see it, it is probably due to a too limited selection.


Finally, the same stone measured on one machine compared to another could well give a different result by 2 to 3%, with the difference becoming higher as the LGF-% increases.


''Ground control to major Tom''

I read in your reply to my other post that LGF can not be interpreted separate from main pav angles. So for this particular stone, with pav angles of 40.8 (40.7-40.9), does that somehow excuse a LGF of 74%?

Now suppose that this stone with the same LGF-angles had a main pavilion angle of 41.0°. With the difference in angle being smaller, the LGF-% would be higher, possibly 76 or 77%, maybe even higher (let others calculate this).

What you did is, you marginally changed a minor part of the stone (is it 25% of the pavilion-surface?), with the major part of the pavilion remaining the same, so essentially the major contributor to light return has not changed.

These two stones, with the same LGF-angle and a different main pavilion angle, are closer to eachother than two stones with the same LGF-% and a different main pavilion angle.

In figures, a stone with 40.8°-angle and 74% LGF-% and a stone with 41.0°-angle and 77% LGF-% are closer brothers than
a stone with 40.8°-angle and 77% LGF-% is to a stone with 41.0°-angle and 77%-LGF-%.

Live long,