Cushion Cut Calculations

[unobtanium]

First I want to thank everyone that is active on Pricescope for their wealth of knowledge. I have spent a great deal of time lurking on the forums and reading up the tutorials. I am shopping for an engagement ring and I know I want to purchase a cushion cut stone. Problem is, I am an engineer and I want to some how quantify the sizing of the stones.

I want to pose a potential calculation scheme for grading the face up size of a CUSHION cut stone. For this I will use a two stone example. Obviously the table % and depth % are large indicators of stone size, but when there are two stones that are very similar this calculation may be useful: (my goal is to verify if my logic is wrong or right)

Stone 1:
Cushion Cut
2.31 Ct
Table 63%
Depth: 68.9%
Dim: 9.09 x 7.31 x 5.04
Cut Grade: VG

Stone 2:
Cushion Cut
2.5 ct
Table 66%
Depth 69.5%
Dim: 7.55 x 7.28 x 5.06
Cut Grade VG

Keeping Color and Clarity constant between the two stones, lets decide which stone will have a larger face up appearance:

Calculations:
FACE area:
L x W= area (mm2)
Table Area:
area x table % = table area

Stone 1:
FACE area:
9.09 * 7.31= 66.4479 mm2
Table Area:
66.4479 * .63 = 41.862


Stone 2:
FACE area:
7.55 * 7.28= 54.964 mm2
Table Area
54.964 * .66= 36.2764



From these calculations it is easy to conclude that Stone 1 will have a much larger face up look and a larger table area look than that of Stone 2. I know that this is all relative to the other Cs, but I feel this is a good tool to evaluate how well the stone will look "face up." Keep in mind that stone 1 is .19 ct less than stone 2, which could mean that you are getting a larger looking stone for less $. (keeping all other things constant) (Yes I know that cushion cuts are not perfectly square and that the area calculation is not exact, but a good comparison point)



This actual sitation exists online, Stone 2 is $300+ more than stone 1.

 

[ChunkyCushionLover]

Date: 12/8/2009 5:22:21 PM
Author:unobtanium

First I want to thank everyone that is active on Pricescope for their wealth of knowledge. I have spent a great deal of time lurking on the forums and reading up the tutorials. I am shopping for an engagement ring and I know I want to purchase a cushion cut stone. Problem is, I am an engineer and I want to some how quantify the sizing of the stones.

I want to pose a potential calculation scheme for grading the face up size of a CUSHION cut stone. For this I will use a two stone example. Obviously the table % and depth % are large indicators of stone size, but when there are two stones that are very similar this calculation may be useful: (my goal is to verify if my logic is wrong or right)

Stone 1:
Cushion Cut
2.31 Ct
Table 63%
Depth: 68.9%
Dim: 9.09 x 7.31 x 5.04
Cut Grade: VG



Stone 2:
Cushion Cut
2.5 ct
Table 66%
Depth 69.5%
Dim: 7.55 x 7.28 x 5.06
Cut Grade VG

Keeping Color and Clarity constant between the two stones, lets decide which stone will have a larger face up appearance:

Calculations:
FACE area:
L x W= area (mm2)
Table Area:
area x table % = table area



Stone 1:
FACE area:
9.09 * 7.31= 66.4479 mm2
Table Area:
66.4479 * .63 = 41.862




Stone 2:
FACE area:
7.55 * 7.28= 54.964 mm2
Table Area
54.964 * .66= 36.2764







From these calculations it is easy to conclude that Stone 1 will have a much larger face up look and a larger table area look than that of Stone 2. I know that this is all relative to the other Cs, but I feel this is a good tool to evaluate how well the stone will look 'face up.' Keep in mind that stone 1 is .19 ct less than stone 2, which could mean that you are getting a larger looking stone for less $. (keeping all other things constant) (Yes I know that cushion cuts are not perfectly square and that the area calculation is not exact, but a good comparison point)



This actual sitation exists online, Stone 2 is $300+ more than stone 1.

I don't agree with the depth calculation method (using width) that is used for fancies, it would be more appropriate to do a point to opposite point calculation (use Pythagoras (IE. Point to Point Width = SQRT(Length Squared X Width Squared) if you did that you would notice that now cushion 1 has a depth of 43%

and Cushion 2 is 48% in depth thats a very big difference.

I suspect given the large tables both of these are

but if you want to be sure ask the vendor for an ASET. Faceup size is not the most important thing with diamonds otherwise we'd all be wearing pieces with 20% depth. Cushions require a large crown height to maximize fire and brilliance from internal reflections.

I don't find your calcultion method of much use, just look at Length/Width Ratio and Depth in Cushions if you want a fair idea of the faceup size, from above you also see crown facets, it is true a larger table will look flatter and spreadier but that is an oversimplification and not what the eye actually sees.

You are trying to compare a rectangular stone with a more square one. I would suspect the light return of Stone 1 would be

but we wouldn't be able to tell without photographs and preferably an ASET.

 

[kenny]

Don't focus too much on face up, or spread.
A flat thin glass slide has superb face up for its weight.

Beauty in a diamond comes mostly from cut - all the facets being positioned so they return maximum light in a pleasing pattern.
This may run counter to maximum face up for the weight.
Besides, a poorly- cut diamond with a greater spread may seem smaller because it is darker.

Learn about good cut.
Cut is the most important C but the least understood.

I used to be an engineer too; but I have been cured.

 

[CharmyPoo]

I am an engineer too but unfortunately it isn't that simple. Our eye is also tricked by edge to edge brillance that will make the diamond look bigger than it is.