Spread vs Carat Size - is there a tool?

[TODiamonds]

I often see the experts on this forum mentioning whether a diamond faces up big or small vs it's carat size.

I've always wondered how you guys determine that - is there a chart or tool online that I can refer to?

I've found a few guidelines for certain popular sizes (1, 1.5, 2.0, etc) but I'm looking for something more precise down to the x.xx carat. As an example, my stone is 1.62 carats and spread is 7.50x7.52 and I would love to determine whether that's average, big or small.

Thanks!

 

[Dancing Fire]

I've found a few guidelines for certain popular sizes (1, 1.5, 2.0, etc) but I'm looking for something more precise down to the x.xx carat. As an example, my stone is 1.62 carats and spread is 7.50x7.52 and I would love to determine whether that's average, big or small.

Thanks!

The diameter looks good. If you wanna check the diameter of a well cut stone just go to one of the super ideal cut vendor's website and compare diameters on the same weight of stone.

 

[TODiamonds]

That's a great idea, thanks!

Is that what you did to determine that the diameter of my stone looks good?

I feel like there MUST be some kind of tool out there... this just boils down to math based on assumed set of ideal proportions after all..

 

[Dancing Fire]

That's a great idea, thanks!

Is that what you did to determine that the diameter of my stone looks good?

Yes, Same weight as your 1.62ct from a super ideal vendor.
https://www.hpdiamonds.com/en-us/diamonddetail/HPD10770

 

[headlight]

This is a great topic.
There are a number of sources online for this data, but there’s a range on what these websites state. It’s very confusing.

 

[sledge]

I guess it depends on everyone's opinion of precision. Even within ideal cut stones, you can get some variance because of the way the proportions play together.

FYI, the HCA calculator does provide a size comparison.

A few examples. Notice how the smaller carat weight sizes up a smidge larger than the larger carat weight? Granted, these stones have two very different personalities; however, looking at ASET, H&A, etc you can see they are both ideal cut stones.

Stone 1 - 0.9ct G VS1, 56 table, 62.4 depth, 36 crown & 40.6 pavilion
https://www.whiteflash.com/loose-diamonds/round-cut-loose-diamond-3945630.htm



Stone 2 - 0.867 H VS2, 57.9 table, 60.1 depth, 33.9 crown & 40.7 pavilion
https://www.briangavindiamonds.com/...0.867-h-vs2-round-diamond-ags-bl-104098623002

 

[TODiamonds]

Thanks guys, it sounds like this is more of a subjective measure and as such there isn't a comprehensive tool out there.

If I knew the math around diamonds better I'd be inclined to program a tool that allows you to compare spread of any sized diamond down to the 0.01 carat against a reference diamond based on predetermined ideal specs (ie: 34.x crown, 40.x pavillion, 60.x depth). I'm guessing once you have enough inputs it's just a matter of math to determine the diameter.

I'll stick to comparing against super-ideal sites like HP, WF, etc for now. Thank you Dancing Fire for that suggestion!

 

[Texas Leaguer]

@TODiamonds,
You are correct that 'spread' is based on math. But certain assumptions are also required in making qualitative judgements about spread. Below is a snippet from an article on grading at AGSL.
For a deeper dive, see the links at the end of the article to the peer reviewed foundational science behind the light performance cut grading system of AGSL.
https://www.whiteflash.com/about-diamonds/diamond-education/ags-cut-grading-scale.htm

Weight Ratio
Assessing the size of a diamond in relationship to its carat weight (also referred to as “spread”) is important in making sure that the diamond has no significant amount of hidden weight. Since diamonds are sold by the carat, and prices increase as diamonds get bigger, there is an incentive for cutters to retain as much weight as possible, especially when close to “magic” marks such as 1.00ct. There are a number of cutting techniques used to accomplish this. By calculating what the weight of the diamond should be according to its dimensions, it is possible to know if hidden weight is significant and to apply the proper penalty for it.
Expected weights are determined according to mathematical formulas. A diamond weighing in excess of 5% more than its expected weight will receive a weight ratio deduction. For example, a round diamond that is 6.47mm is expected to weigh 1.00 ct. If it weighs more than 1.05ct, it receives a weight ratio deduction.