Lower Half and Star Lengths

[teknikal]

Hello all,

I recently bought an engagement ring for my girlfriend and so my search is over.
However, being the geek that I am... I get more and more curious about diamonds.
I never really quite understood the implications of 'Lower halfs' and star lengths.
Crown and pavilion angles I get.. mirrors and prisms... but lower half and star lengths? How do they affect light performance?

For example... the diamond I chose for my girlfriend is slightly steep/deep... with 35.5/40.8. It is triple excellent GIA but as far as I understand it won't be an AGS 0.
I'm very happy with how it looks and so I'm keeping it. Fine by me. And in a week time...I'm pretty sure my girlfriend will too

Now I read somewhere in the depths of price scope that for that specific combination...35.5/40.8...the lower half percentage matters a lot...but noone seems to be able to explain how

my diamond has a 75% lower half and a 50% star length.. not cutlet..and medium girdle.

How would the same 35.5/40.8 diamond perform with a 80% lower half...and a 45% star length... for example?

Essentially, what do these two things do???

 

[Rhino]

Hi tek,

Great questions.

Hello all,

I recently bought an engagement ring for my girlfriend and so my search is over.
However, being the geek that I am... I get more and more curious about diamonds.
I never really quite understood the implications of 'Lower halfs' and star lengths.
Crown and pavilion angles I get.. mirrors and prisms... but lower half and star lengths? How do they affect light performance?

HUGE. After the pavilion mains the lower halves are what I would consider the second most essential facet set dictating not only light performance but personality of the diamond. Primarily lower halves and stars to a lesser degree will impact the *nature* of a diamonds scintillation. Both patterned and sparkle. I've shot a video specifically on this very subject covering lower half lengths from 60% upwards of 85%.

For example... the diamond I chose for my girlfriend is slightly steep/deep... with 35.5/40.8. It is triple excellent GIA but as far as I understand it won't be an AGS 0.
I'm very happy with how it looks and so I'm keeping it. Fine by me. And in a week time...I'm pretty sure my

Now I read somewhere in the depths of price scope that for that specific combination...35.5/40.8...the lower half percentage matters a lot...but noone seems to be able to explain how

my diamond has a 75% lower half and a 50% star length.. not cutlet..and medium girdle.

How would the same 35.5/40.8 diamond perform with a 80% lower half...and a 45% star length... for example?

Essentially, what do these two things do???

It's against forum rules to post my own videos here but it shows you precisely what you're asking to see. To put it into vernacular ... the longer the lower half length the more you decrease broad flash and increase pin flash. Vice versa the more you shorten lower half length, the more you increase broad flash and decrease pin flash.

Hope that helps.

Kind regards,
Rhino

 

[teknikal]

Hi Rhino,

Interesting.
It's funny then that the guidance on Tolkowski ideal cut ranges dont include lower half percentages.

What's 'better' - 75% or 80%? When are they 'better' in your experience?

Thanks!

ps: will hunt the video. I'm assuming GoodOldGold on youtube?

 

[Rhino]

Hi Rhino,

Interesting.
It's funny then that the guidance on Tolkowski ideal cut ranges dont include lower half percentages.

What's 'better' - 75% or 80%? When are they 'better' in your experience?

Thanks!

My pleasure. Actually neither is technically "better". It's truly a matter of taste at that point. Some people prefer the thinner/smaller pin fire flash of 80 or even 85% (which we've actually cut for people purposely) or the broader flash of 75% or even BROADER than that in the August Vintage European cuts which specialize in broad flash with Ideal optics. When you're talking about diamonds with ideal optics there are a variety of flavors available.

All the best,
Rhino

 

[Texas Leaguer]

Hi Rhino,

Interesting.
It's funny then that the guidance on Tolkowski ideal cut ranges dont include lower half percentages.

What's 'better' - 75% or 80%? When are they 'better' in your experience?

Thanks!

ps: will hunt the video. I'm assuming GoodOldGold on youtube?

From a standpoint of taste, what appeals in terms of the scintillation is pretty much in the eye of the beholder. A couple of things to remember: 1) GIA rounds lower halves to the nearest 5%. So the difference between a GIA 75 and 80 could be much smaller than you might think. 2) One of the things that makes a well cut round such a universally admired gem is it's balanced mix of virtual facet sizes. So, getting too long and splintery on the lowers shifts that balance to the small side. This too implies that the overall size of the diamond is something to consider, as is facet precision (sometimes referred to as optical symmetry).

 

[teknikal]

Thank you TexasLeaguer

If I'm referring specifically to my diamond then. I see that it has a 75% lower half. and 50% star length. it is a 1.16 carat diamond with 6.73x6.77 diameters. So it's a 'shorter' lower half I gather

... what am I looking for here? I'm trying to relate the numbers to tangible observations.

If it was an 80%, what would I see differently on my diamond?

Thanks all

 

[Rhino]

ps: will hunt the video. I'm assuming GoodOldGold on youtube?

Just type in lower half facets. Should be #1 in your search.

 

[Texas Leaguer]

Thank you TexasLeaguer

If I'm referring specifically to my diamond then. I see that it has a 75% lower half. and 50% star length. it is a 1.16 carat diamond with 6.73x6.77 diameters. So it's a 'shorter' lower half I gather

... what am I looking for here? I'm trying to relate the numbers to tangible observations.

If it was an 80%, what would I see differently on my diamond?

Thanks all

Your diamond will produce some larger sparkles in the mix than if it had lower halves closer to 80. As the lower half facets shorten the pavilion mains are allowed to be bigger, and as the lowers lengthen the mains get progressively narrower.

Regarding the 3D facet precision, this image of virtual facets illustrates how the sparkles are varied in size in a classic round brilliant. It also implies that in order to get the specific mix intended by the facet design, precision alignment in 3D is critical. High precision also results in very symmetrical patterns of contrast and sparkle that are particularly pleasing to the eye.

 

[EvaEvans]

I personally prefer smaller lower halves, so 75% is just fine.