confused about face up size - cushion vs. radiant

[lulu3]

Hello. I have read a lot on this board about cushions face up size being smaller than for example radiants. So out of these two cushion stones:

2.18 carat; 8.31x7.42x4.80mm
2.22 carat; 7.58x7.44x5.08mm

the first cushion would face up larger correct? How do you determine if a cushion stone is bottom heavy, which makes it look smaller? Is it these measurements that tell you where the weight is? so the larger carat stone has more weight in the bottom and less in the dimensions so it appears smaller?

And when you compare a cushion to a radiant, would you use the same measurements of the radiant stone to see which one would face up larger, as compared to the cushion?

thank you in advance for any help.

 

[kittykat717]

Well from what I can tell, it seems to do a lot with how it is cut. If it is cut too deep, it's going go affect the diameter and appear smaller than it really is. And it would have the opposite affect if it is a shallow cut. This explains what I am talking about:

http://www.diamondinfo.org/content/diamond-carat-weight.php

 

[slg47]

yes look at the mm measurements. the first one is much more rectangular.

 

[lulu3]

so based on what dimensions do you determine if its bottom heavy or not?

 

[oldminer]

Bottom heavy is not a useful term. What you are thinking about is the Total Depth from table to Culet. The thicker (deeper), the stone, the more weight is trapped in the vertical axis. It does not really matter if the depth is greater in the bottom (pavilion), the top (crown), or the Girdle Thickness. Usually extra total depth it is shared by all three or at least two of the above. Regardless, extra depth results in reduced visual size within the outline of the plane of the girdle. In a cushion, this can also be altered by greater or lesser width or degree of roundness of the 4 corners of the stone. They can be broad 1/4 circles or rather tight 1/4 circles in radius and that will have an effect on the actual square mm of visible size. One can generalize that the best cut stones have a moderate depth of their pavilions and also a moderate total depth. Too thin is just as bad for cut quality as too deep. The results are not the same from thinness vs deepness, but neither is better than moderation according to the visual optical properties of diamond which are not ever changed by cutting. Cutting has to work within the confines of how diamond refracts light.

 

[lulu3]

thank you. But I still do not understand how to be able to compare a cushion with a radiant and figuring out which one faces up larger. What dimensions should I be using and what calculation? I read a really great post about it once but cannot find it anymore. It was very easy to understand.

 

[Rockdiamond]

It really needs to be a visual inspection.
Sometimes the bottom of a stone can be a little "swollen"- it's called pavilion bulge and not indicated in measurements.
But you can use the LxW to get a better idea of face up size

 

[Dreamer_D]

Lulu if you want to know face up size, multiple l x w, it gives an estimate of the area of the face. The first is 62, the second 56 square mm. So the first will look larger. Belly weight does not matter for what you want to know since you have the face up size. It matters more when trying to figure out if you are getting good face up size for your dollar! But I would focus more on optics and finding a stone that look great than getting too caught up in each little square mm of face up size. Will you see these in person? Which type of light cut do you prefer? Cushions and radiants can look very different.

 

[rubybeth]

thank you. But I still do not understand how to be able to compare a cushion with a radiant and figuring out which one faces up larger. What dimensions should I be using and what calculation? I read a really great post about it once but cannot find it anymore. It was very easy to understand.

The one with the larger dimensions faces up larger, so it's an easy comparison between two square stones:

2.18 carat; 8.31x7.42x4.80mm - multiply 8.31 x 7.42 = 61.66
2.22 carat; 7.58x7.44x5.08mm - multiply 7.58 x 7.44 = 56.39

Face up size is how much area the diamond will cover when set in a piece of jewelry. Think of the diamond not as a 3-dimensional object but as though it is flat (forget about depth--in this case, the 4.80mm and 5.08mm figures). The top of the diamond is the only thing that matters when comparing face up sizes, so the depth isn't important to figure this out. You could even cut out little pieces of paper to compare what 8.31x7.42mm looks like vs. 7.58x7.44mm.

If you really want to compare how different cuts face up, try looking at a pear and a princess. You get a nice big face up size with a pear for the carat weight compared to a princess, because pears can be cut much more 'shallow' than square cuts.

 

[Dreamer_D]

Depth matters to the extent that is affects carat weight. Face up size is good, but you also don't want to pay a whole bunch for carat weight that is not required. That is why people bring up depth as a negative sometimes.

You could, for example, divide the face up mm by the carat weight to see how much spread per carat you are getting! In the case of the 2.18ct stone, you are "buying" 28.28 square mm per carat. In the case of the 2.22 you are "buying" 25.40 face up square mm per carat. So in a sence, you are paying for carat you cannot see with the 2.22 Its depth is less, so it is not hiding weight in the depth, my guess is it has a thicker girdle?

So from that purely financial perspective, the 2.18 wins. And from a face up spread perspective it wins. But those are much less important factors IMO than how the diamond performs and looks in real life, assuming the prices of these two diamonds are comparable.

 

[oldminer]

Multiplying length times the width is a simple, but partial solution. It does not take into account the curvature of a cushion nor the cut corners on a radient cut. It will give you something factual, but may not be totally correct. If you want the exact square mm of the enclosed plane of the girdle, you will need someone with a high quality diamond scanning device such as a newer Sarin, a Helium, Ogi or ImaGem to provide the exacting data. Some folks want approximations. Others want exactness.