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can someone please explain what stars do?

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Laila619

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Hi,

I understand LGF and how they affect a RB but not really what stars do. What range of stars is ideal? 52 + ? Is there a simple way to understand what they do? Like for instance, I understand that the longer the LGF, the thinner the arrows, and thus the thinner the flashes of light (ie more pinfire instead of large flashes). Is there an equivalent with star facets? Thanks!
 

jet2ks

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FB.

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Stars twinkle.
31.gif
 

Laila619

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I've read that tutorial but still don't quite get it. Jon says they make more hot spots, so I'm assuming the bigger the stars the more sparkle in the stone?

But then I read once where I believe John Pollard said on this forum that stars are best when 53 or under.
 

Lorelei

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Date: 11/13/2009 11:49:16 AM
Author: Laila619
I've read that tutorial but still don't quite get it. Jon says they make more hot spots, so I'm assuming the bigger the stars the more sparkle in the stone?

But then I read once where I believe John Pollard said on this forum that stars are best when 53 or under.
You have the idea, yes - the star facet windows being ' opened' will allow more light to show through, you will see more ' sparks' from those areas. Rather like window shutters I suppose as an analogy, open your shutters and that allows more light to be seen, close them and you see less.

I will edit the text from Jon to try to elaborate further. My comments in bold.

"The star facets are the windows to the hot spots (blacks observed in LightScope analysis) between the arrow heads which are directly influenced by the lower girdle facet length. You can see these and they resemble almost watermelon seeds between the arrow heads on the IS images. The star facets are related to the length of the lower girdle facets. When diamonds have proper lower girdle facet length ( in other words LGF length which works well for the diamond), opening these windows increases the *sparks* that fly out from between the fiery arrows, thus increasing both fire and scintillation as observed in direct light conditions. In other words suitable lower girdle facet length helps ' open' the star facet windows, allowing these areas to pop with fire and scintillation. Closing these windows decreases that optical effect so the situation would be reversed. GIA's most recent research shows exactly what I've been discovering concerning this phenomena. Here are examples of diamonds with varying star/upper girdle ratios." See diagrams.





 

stone-cold11

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Similar to the relationship between the crown and pavilion angles.
 

Laila619

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I found the quote from John Pollard I was looking for:

"Our prescriptions for stars exist to eliminate leakage near the girdle. Remember that the stars determine the length of the upper girdles. Stars too long result in short upper girdles with upper girdle angles too steep. Stars too short have the opposite effect, and the lower girdles are too long. The 48%-51% configuration is the best combination for us with regard to balance between stars and uppers and our ability to minimize leakage and maximize performance."

What I don't get is that if stars make for more sparkle why would you want to keep them at 48-51% only?
33.gif


Stone-cold, so if it's similar to the crown/pavilion angle relationship then if a stone has long LGF, do you want short stars? And vice versa? Short LGF/bigger stars?
 

Lorelei

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Date: 11/13/2009 12:06:25 PM
Author: Laila619
I found the quote from John Pollard I was looking for:

'Our prescriptions for stars exist to eliminate leakage near the girdle. Remember that the stars determine the length of the upper girdles. Stars too long result in short upper girdles with upper girdle angles too steep. Stars too short have the opposite effect, and the lower girdles are too long. The 48%-51% configuration is the best combination for us with regard to balance between stars and uppers and our ability to minimize leakage and maximize performance.'

What I don't get is that if stars make for more sparkle why would you want to keep them at 48-51% only?
33.gif


Stone-cold, so if it's similar to the crown/pavilion angle relationship then if a stone has long LGF, do you want short stars? And vice versa? Short LGF/bigger stars?
I think John meant that as applied to the Infinity brand Laila, not in general.
 

stone-cold11

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I think that star is cut to aim for the contrast effect you see in pavilion main, the arrow shafts. else it would just be a light return surface. So I am guessing a larger lower half complements a smaller upper half, to get the same effective angle difference between these 2 facets as the crown and pavilion mains, the range I am not sure.
 

John P

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Date: 11/13/2009 12:12:08 PM
Author: Lorelei




Date: 11/13/2009 12:06:25 PM
Author: Laila619
I found the quote from John Pollard I was looking for:

'Our prescriptions for stars exist to eliminate leakage near the girdle. Remember that the stars determine the length of the upper girdles. Stars too long result in short upper girdles with upper girdle angles too steep. Stars too short have the opposite effect, and the lower girdles are too long. The 48%-51% configuration is the best combination for us with regard to balance between stars and uppers and our ability to minimize leakage and maximize performance.'

What I don't get is that if stars make for more sparkle why would you want to keep them at 48-51% only?
33.gif


Stone-cold, so if it's similar to the crown/pavilion angle relationship then if a stone has long LGF, do you want short stars? And vice versa? Short LGF/bigger stars?
I think John meant that as applied to the Infinity brand Laila, not in general.
I recall that range as an 'A Cut Above' bullseye, so the quote is likely from a JohnQuixote post specific to that design.

In the big picture of all possible configurations every measurement relies on the collective, so there's no single "ideal" range, to me, without knowing the other proportions. The target is often fine-tuned based on lower halves and table size. Remember also that when blocking the crown for CA as the priority with normal indexing, adjusting the stars one way or another in brillianteering will change the upper break facet angles. That's a give and take which is important to consider.
 

Karl_K

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The stars themselves have little effect the change in the upper girdle facet angles is a much bigger effect.
Longer stars and steeper upper girdles can be an advantage on shallower crowns and a disadvantage on steeper crowns.
With a 33-34.2 degree crown and 54-57 tables I like 60 stars but it isn't that huge a deal.
With a 35+ crown it is better to stay close to 50 or under.
In the 34.2 to 34.9 range with 54-57 tables it don't matter much.
 

Laila619

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Date: 11/13/2009 4:48:10 PM
Author: Karl_K
The stars themselves have little effect the change in the upper girdle facet angles is a much bigger effect.
Longer stars and steeper upper girdles can be an advantage on shallower crowns and a disadvantage on steeper crowns.
With a 33-34.2 degree crown and 54-57 tables I like 60 stars but it isn''t that huge a deal.
With a 35+ crown it is better to stay close to 50 or under.
In the 34.2 to 34.9 range with 54-57 tables it don''t matter much.
Hi Karl,

Well the upgraded diamond I bought has a slightly steep crown at 35.2 and 52 % stars. Table is 56.8. Is that ok?
 

Karl_K

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Date: 11/13/2009 5:15:31 PM
Author: Laila619
Date: 11/13/2009 4:48:10 PM

Author: Karl_K

The stars themselves have little effect the change in the upper girdle facet angles is a much bigger effect.

Longer stars and steeper upper girdles can be an advantage on shallower crowns and a disadvantage on steeper crowns.

With a 33-34.2 degree crown and 54-57 tables I like 60 stars but it isn't that huge a deal.

With a 35+ crown it is better to stay close to 50 or under.

In the 34.2 to 34.9 range with 54-57 tables it don't matter much.

Hi Karl,


Well the upgraded diamond I bought has a slightly steep crown at 35.2 and 52 % stars. Table is 56.8. Is that ok?
yes it is fine.
55% would likely be ok also depending on lower girdles and pavilion angle.
It is one of those things that the IS shows if its an issue and numbers don't tell the whole story.
 

stone-cold11

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Well, it is above 35 CA and star is around 50%, so I guess you are safe. :razz:
 

Stephan

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In a lot of pavilion/crown combos, GIA cut grading system penalizes long star facets when they go together with long lower girdles. Such diamonds often show a lot of black (obstruction) inside the table (between the arrows) when looked with an IdealScope.
But GIA seems to like long star facets with short lower girdles. In some steep/deep combos, this can cause leakage.
As Karl says, the important thing when talking about star facets is the impact they have on upper girdle angle and as Jonathan mentions in his interesting tutorial, above a certain upper girdle angle, light return will decrease.
One of my favorite combos for nice fire display under strong lighting (halogen) is a diamond with excellent HCA (one of the combos with slightly shallow crown) and long star facets (and of course a decent IScope picture).
I think there is something exciting in finding diamonds (like the one that is on hold for me right now) that score well on the HCA and with upper girdle angle higher than the lower girdle angle (unlike the crown angle that is generally lower than the pavilion angle) but I know there is nothing scientific in my approach.
The diamond that is on hold for me now has 57% table, rounded 50% stars because the crown is steep (rounded 35.5). The pavilion is rounded 40.6 with rounded 80% LGF. If the IdealScope picture is nice, I'll post more information.
 

Khalsa007

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QueenMum, I''d really like to see that IS image if possible.

Those are the exact dimensions of my diamond that I just purchased. I just saw it through the IS viewer but I don''t recall now how it looked, I''m pretty sure there was little to no leakage. The symmetry received a very good and the hearts and arrows were perfectly viewable.

I just started learning about stars and LGF now (should completely finished my hw before I went to buy the diamond but I think it''ll be alright)

Table is 57%, Stars 50%, Crown is 35.5%, Pavilion is 40.6% and LGF is 80%. I used the HCA while I was in the store on my iPhone and it came out to be 1.4 within TIC. I''m worrying about nothing right? haha..
 

tonyc2387

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Date: 11/13/2009 5:35:58 PM
Author: Karl_K
It is one of those things that the IS shows if its an issue

I know Lorelei quoted from Jon''s website about where to look on an IS... but do you happen to have any examples of an IS that you would classify as ''having issues'' vs ones that don''t?
 

Karl_K

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Date: 1/25/2010 6:01:32 PM
Author: tonyc2387
Date: 11/13/2009 5:35:58 PM

Author: Karl_K

It is one of those things that the IS shows if its an issue


I know Lorelei quoted from Jon''s website about where to look on an IS... but do you happen to have any examples of an IS that you would classify as ''having issues'' vs ones that don''t?
When you start getting more leakage than normal under the upper girdles is when it is an issue.
I somewhat disagree with Jon''s article on the issue.
It really depends on the c/p combo and it is a very very small difference if any that the stars make themselves.
As I said earlier the upper girdle angles can and do make a difference with some crown angles which is related to star length.
For me the stars are not a huge issue as long as the upper girdle angle is good.
 

Lorelei

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Date: 1/26/2010 1:47:24 AM
Author: Karl_K

Date: 1/25/2010 6:01:32 PM
Author: tonyc2387

Date: 11/13/2009 5:35:58 PM

Author: Karl_K

It is one of those things that the IS shows if its an issue


I know Lorelei quoted from Jon''s website about where to look on an IS... but do you happen to have any examples of an IS that you would classify as ''having issues'' vs ones that don''t?
When you start getting more leakage than normal under the upper girdles is when it is an issue.
I somewhat disagree with Jon''s article on the issue.
It really depends on the c/p combo and it is a very very small difference if any that the stars make themselves.
As I said earlier the upper girdle angles can and do make a difference with some crown angles which is related to star length.
For me the stars are not a huge issue as long as the upper girdle angle is good.
In the proverbial nutshell. Its good to know what stars do but they are of less significance than other factors concerning the beauty and optics of a diamond.
 
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