Stars and lower girdle facets

[cj2be]

hello all! I’ve tried to do some research from previous threads to see if I can get an answer to my question but nothing is spot on. I came across this article and I have a question as to its validity:
https://www.prosumerdiamonds.com/lower-girdles/

Are 60% stars with 80% lgfs always a deadly combination? If so, how? And as always, any information regarding the relationship between stars and lower girdles is welcome! I love learning about the intricacies of diamonds and I really appreciate everybody’s insight

 

[Karl_K]

huge heaping dump of internet drivel.
Which range of stars that work well depends on the table size, crown angle and the presence of digging or painting as well as the pavilion angle to a slightly lesser degree.
With a steep crown shorter stars will help lower the upper girdle angle.
Same with a large table.
However with a shallower crown it will steepen the upper girdle angle aiding in scintillation.
Same with a smaller table at times.

On of the nicest diamonds I have seen that for my preferences rocked and rocked hard was a tight and optically symmetrical 60% stars and 80% lowers with a 55t/34/41 combo.

 

[cj2be]

Just what I was looking to hear!! My 60 star 80 lgf is a 57t/35/40.8 combo. Have you seen any like that before??

huge heaping dump of internet drivel.
Which range of stars that work well depends on the table size, crown angle and the presence of digging or painting as well as the pavilion angle to a slightly lesser degree.
With a steep crown shorter stars will help lower the upper girdle angle.
Same with a large table.
However with a shallower crown it will steepen the upper girdle angle aiding in scintillation.
Same with a smaller table at times.

 

[Karl_K]

Just what I was looking to hear!! My 60 star 80 lgf is a 57t/35/40.8 combo. Have you seen any like that before??

GIA numbers?

 

[Karl_K]

Just looked at the pics in the other thread its fine, no issues.
Why I asked if they were GIA numbers is GIA grossly rounds everything.
If it was an issue it would show up in your pictures.

 

[Diamond_Hawk]

Why I asked if they were GIA numbers is GIA grossly rounds everything.

^^This. Yes. A million times yes.

 

[cj2be]

Very surprised that it has a bruted girdle. But Im coming to find I like the old school touch

GIA numbers?

 

[Karl_K]

That combo in actual numbers not gia numbers is right on the edge of where the uppers can get over steep with the long stars but there is no sign of it with your stone.
Your good to go.

 

[yssie]

This fellow started his site by parroting various PSers’ posts, professional and consumer, sans both context and attribution. He has a long history of making sweeping statements backed up with florid pseudo-logic.

I have confidence in neither his intentions nor his execution.

 

[cj2be]

That combo in actual numbers not gia numbers is right on the edge of where the uppers can get over steep with the long stars but there is no sign of it with your stone.
Your good to go.

Do you have any links you can give me so I can read up some more on this?!

 

[Karl_K]

Do you have any links you can give me so I can read up some more on this?!

I have covered it in scattered posts over the years, off the top of my head I dont know of what I would consider a good write up on it.

 

[Karl_K]

Here is what it looks like to have over steep lowers. The arrows point to it and the green circles it, it is all around the stone 8 times.

 

[DiamondsAndDior]

This is really interesting and something I did not know about. What type of stars and LGF numbers would work best with steep crown angles and smaller tables?

 

[Karl_K]

This is really interesting and something I did not know about. What type of stars and LGF numbers would work best with steep crown angles and smaller tables?

The small table offsets the need for shorter stars up to a point.
The distance between the table and girdle is longer there for the uppers will be longer and at a lower angle for any given star %.
Many OEC style stones with really steep crowns were cut with 45% range stars.

With a MRB you will generally see 50%.

Where they become super critical and you most often see problems is with a larger table and steep crowns.
I have seen more 60/60 style stones with upper girdle angle issues than any other combos.
The upper girdles are right over the lower girdles so lower girdle angle makes a difference in what upper girdle angles work.
Since the lower girdle angles are set by the pavilion angle and the lgf% when there is min. painting or digging it gets real complicated.
What works for a 41 pavilion 80% lowers may not work for a 40.7/75% lowers.

As far as the lowers since a shallower pavilion is a match to steep crowns in general longer lowers for reduced obstruction under the small table(smaller arrows) is a good match. Then match the uppers angles to the lowers angles to make sure you dont get a problematic match.
80% gia, 78+ real numbers. It really depends on the specifics of the actual angles and personal preference.
Or go the other way and make it an oec with short lgf% in the 65% range with the lowers ending right at the edge of the table.
GIA creates a real mess with this with all the rounding.

 

[bmfang]

as usual, @Karl_K comes up with the technical goodies to explain to the rest of us!

 

[cj2be]

Here is what it looks like to have over steep lowers. The arrows point to it and the green circles it, it is all around the stone 8 times.

What is it normally supposed to look like??

 

[Karl_K]

What is it normally supposed to look like??

I'm not at my computer to make an image but the areas i pointed out(upper girdles) wouldn't overall be darker.

 

[Karl_K]

This is a virtual images of a stone with upper girdles that are not overly steep in the same virtual lighting.
Notice while some of the upper girdles are darker from the lighting it does not have the scalloped look of the one with issues.
Once you have seen it in person you cant mistake it.

 

[cj2be]

Why I asked if they were GIA numbers is GIA grossly rounds everything.

^^This. Yes. A million times yes.

Speaking of rounding, I’m not sure if you can tell by the picture, but do my lower girdles look more like 78% or 82%?

And assuming they round stars as well, do my stars look like they are under 60 or over 60?

 

[OoohShiny]

LGFs / arrow shafts look pretty narrow to me - I'd be tempted to say they are towards the longer end of the scale.

 

[bmfang]

LGFs / arrow shafts look pretty narrow to me - I'd be tempted to say they are towards the longer end of the scale.

I’m thinking LGF of 78-79 based on the IRL pics.

 

[Karl_K]

Speaking of rounding, I’m not sure if you can tell by the picture, but do my lower girdles look more like 78% or 82%?

And assuming they round stars as well, do my stars look like they are under 60 or over 60?

From a picture I can get pretty close with a lot of work.
But before I do so are you asking because you read more internet drivel or are you just curious?
No matter the numbers your diamond is beautiful.

 

[cj2be]

From a picture I can get pretty close with a lot of work.
But before I do so are you asking because you read more internet drivel or are you just curious?
No matter the numbers your diamond is beautiful.

I'm just curious!! and lately it seems my curiosity has resulted in me reading internet drivel

 

[Karl_K]

I'm just curious!! and lately it seems my curiosity has resulted in me reading internet drivel

lol ok stop reading drivel
As close as I can get and there is some room for error:
Im fairly confident on the stars within +/- 1%
The stars are right at ~60 all the way around.
The lowers are 79-80ish as near as I can tell.
The problem is the overlay in the software I use is of a perfect "diamond" that does not exist in the real world.
So you have to use some fudge factors when aligning it with a less then perfect picture of a diamond and getting all the tilt lined up and no real diamond is perfect.