Lower girdle length

[kmoro]

i’ve noticed that many PSers prefer a lower girdle
length of 75.

However, when I look at diamond education videos, they seem to favor higher numbers - some specifically cutting them to 82. Also, Whiteflash ACÁ specs are 76-80. It looks like there is more sparkle and scintillation with longer lower half lengths, although the flashes are less broad, and it seems to be preferable, at least to the people in the videos.

So my question is: what is up with the 75 preference?

 

[Karl_K]

Some info you might find interesting:
https://www.pricescope.com/articles/do_pavilion_mains_drive_light_return_modern_round_brilliant

Its a balancing act the lower the % the wider the mains(arrows) and you get slightly bigger flashes off the mains.
Like anything else there is no free lunch and this at the expense of less and smaller mid sized flashes between the mains(arrows).
There are some angle combinations that work better with short or long lgf% and some combinations that can work well with anything between 75-80 and likely 70 and 85 if you want to do what some consider extreme in a mrb.
77-78 is nearly universal working well with the widest range of combinations but gia calls 77 75 and 78 80. insanity.

 

[kmoro]

Some info you might find interesting:
https://www.pricescope.com/articles/do_pavilion_mains_drive_light_return_modern_round_brilliant

Its a balancing act the lower the % the wider the mains(arrows) and you get slightly bigger flashes off the mains.
Like anything else there is no free lunch and this at the expense of less and smaller mid sized flashes between the mains(arrows).
There are some angle combinations that work better with short or long lgf% and some combinations that can work well with anything between 75-80 and likely 70 and 85 if you want to do what some consider extreme in a mrb.
77-78 is nearly universal working well with the widest range of combinations but gia calls 77 75 and 76 80. insanity.

Thanks!

That last part about GIA ... I knew they rounded, but I didn’t know they rounded incorrectly!

 

[Karl_K]

Thanks!

That last part about GIA ... I knew they rounded, but I didn’t know they rounded incorrectly!

sorry typo now fixed

 

[Diamond_Hawk]

Here are some nice example of what @Karl_K is referencing.

You see the "fatter" arrow pattern represented by 78 than by 82 as will as the "fatter" hearts.

All of these would be called 80 with GIA rounding, but you could expect different types of performance.



So the combinations are really key. A 78 (called an 80) and a 77 (called a 75) may perform nearly identically (all other parameters being equal). So, broad generalities can be made - but researching each aspect of diamond is key to determining the individual personality inherent in the stone.

 

[rockysalamander]

GIA's rounding is truly mystifying.