Smaller diamonds / lower colours - go for longer LGFs?

[OoohShiny]

Hello again all

I have been reading up tonight on Lower Girdle Facet lengths and the impact they have on the look of the diamond.

Taking them on their own for the moment (and ignoring differences in other facets that will have an impact) it seems the general rule is the shorter the LGFs (<77%) the broader the arrows and the broader the flashes of fire from them due to their increased size, although there won't be so many flashes. At the other end, with longer arrows (>77%, with over 85% being extremely long) the narrower the arrows and the more pinpoint flashes of white that will be given off, with reduced fire due to the light being reflected as a whole back to the eye (rather than being split up into its different colours over a longer length and the eye only seeing one colour out of the many that makes up white light).


Going from the above, if one has a lower colour stone should one look for longer LGFs so that more white light is returned, making the stone look whiter and like a higher colour?

Also, if one is looking for smaller stones, such as earring studs that will have less surface area to reflect light and proportionally smaller facets overall due to its overall smaller size, should one also look for longer LGFs in order to increase the amount of light return from the limited surface area (because there are more flashes than would be the case with shorter LGFs and their fatter arrows, therefore it will look brighter)?


I know a lot is down to personal taste but I just wondered if the science backed up my random thoughts lol

(I guess my main concern is that small earring studs might look a bit 'dead' due to their smaller size and less chance of returning light to a viewer's eyes??! )


If I am completely wrong then I won't mind being told lol

 

[Karl_K]

Your wrong right and in between.
In other words it is not that simple.
Start here:
https://www.pricescope.com/journal/do_pavilion_mains_drive_light_return_modern_round_brilliant

I like slightly shorter lgf in smaller diamonds say under .25 because it gives them just a bit larger flashes off the mains. Certainly 85 actual not gia rounded is too long out to say .75ct, while with the right crown/pavilion 80 or even 81ish is fine in smaller diamonds than that. That is my opinion!

A lot of the fire from a modern RB are not off the mains but from light reflecting from the lower halves so the correlation to fire is not a direct one. It depends on the specifics.
A lot of the time it comes down to personal preference but it can make or break some angle combinations.

 

[OoohShiny]

lol Why am I not surprised it's not that simple, I thought these were just rocks cut to x% of a certain target shape, yet all over the forum we're talking about physiology, angles of light, etc etc...

It really does seem as though there are near-infinite combinations and each has to be assessed individually to ascertain its exact qualities!


Thank you very much for the input, though I understand the concept that shorter LGFs may well give larger flashes, which could be a good thing in a small stone, so it has put my mind at rest regarding some of the stones that I've been considering which have slightly shorter LGFs

 

[teobdl]

The most important thing is getting a diamond with LGF's that pair with that CA/PA/Table %.
That said, if the facets are already small--why make the virtual facets even smaller? If the choice is between well cut and proportioned 75 vs 80 LGF's I'd choose the 75, but wouldn't bend over backwards to get it.

 

[OoohShiny]

Thanks for your reply


I think I still have a LOT to learn regarding the interactions between the different facet proportions so I'm not quite sure how to ascertain if a certain LGF length works with a given stones other measurements but I think I agree that increasing size of reflecting facets in a small diamond might well be a good thing if it gives more fire!

 

[Karl_K]

gives more fire!

How would you define more fire?
More flashes?
Larger flashes?
Brighter flashes?
Quicker flashes that flash on and off quickly as you rotate the stone?
Slower flashes that stay on longer as you rotate the stone?
Shows fire across a broader range of lighting?

multiple choice is fine and there are no wrong answers but explanations are nice :}

 

[teobdl]

Shorter LGF's will give (slightly) fewer but bigger flashes. The kind of light return more dependent on other factors. This website has an excellent tutorial on how everything works together with LGF's in creating more fire or more brilliance http://www.prosumerdiamonds.com/lower-girdles/

 

[Karl_K]

Shorter LGF's will give (slightly) fewer but bigger flashes. The kind of light return more dependent on other factors. This website has an excellent tutorial on how everything works together with LGF's in creating more fire or more brilliance http://www.prosumerdiamonds.com/lower-girdles/

Just a quick look at that page there are several errors or over simplified assumptions.

For example changing the lgf% for any given pavilion angle changes the angle of the lower girdles.(min. painting and digging assumed)
See my article I linked above on how that effects the area under the table. It also effects under the crown lgf interaction changing the angle relationship. Some angle combos are more effected by this change than others.
In general a near modern tolk (56t/34.5/40.75) and around that range is more tolerant to a range of lgf angles than ones farther away from it.
Once you get out past 41 degrees on the pavilion the acceptable range can get very tiny.
Table size can also make a difference because a larger table makes the arrow shaft a larger part of the contrast pattern of the diamond.
In general the smaller the table the shorter the lgf can be without providing too much contrast.
Which is one of the reasons that an oec design works best with a smaller table.

A shorter lgf% can while increasing the size of the flashes off the mains, can decrease the overall average size of the flashes and their visibility.
Like many things with diamonds it is one of those things that is considered a rule does not always hold true.

 

[OoohShiny]

Many thanks for the replies I think I have just about made sense of everything in my head for the moment!

I think I quite like chunkier flashes with a bit more colour, so related to the other discussions that have been taking place on the forum around whether or not you can have a diamond with maximised brilliance but also maximised fire (if I understand it correctly? ), I think I would prefer a bit less brilliance and a bit more fire!

gives more fire!

How would you define more fire?
More flashes?
Larger flashes?
Brighter flashes?
Quicker flashes that flash on and off quickly as you rotate the stone?
Slower flashes that stay on longer as you rotate the stone?
Shows fire across a broader range of lighting?

multiple choice is fine and there are no wrong answers but explanations are nice :}

I think I would refer to big flashes of colour as Fire - I hadn't really thought about the different things in that list there is so much to take in!!

I particularly like some of the Fire videos of the Octavia Asschers, they are pretty amazing