Can you use surface area to compare size of two diamonds?

[nukezero]

I like to know if it's good practice or safe practice to use surface area calculation to determine face-up size view?

for example, I like the size of 1.5 carat round brilliant (most are 7.47 x 7.47mm). So if we take this dimension and find the surface area (face up view), we would get: A=pi * r^2 = (7.47/2)^2 * pi = 43.803mm^2.

Now my girlfriend likes cushion cuts. To achieve the same look, I found a cushion cut that is 1.40 carat, 6.88 x 6.61. But this is cushion so it's more square. Therefore, we can just do length * width = 45.4768mm^2.

So even though the round brilliant is larger in diameter, would the cushion cut look bigger (from a face up perspective) just because it has more surface area mass?

Thanks

 

[ecf8503]

Re: Can you use surface area to compare size of two diamonds

Can you cut them out of a piece of paper and look at them? I would think the round, being larger in all directions, would still look larger, as the cushion could basically fit inside it? Also cushions have rounded corners... so you're losing some area there...

Also - is the cushion rectangular and what vendor is it from? I only ask because BG has a weird way of listing dimensions as length x diagonal instead of the traditional length x width, so even though the stone is square it appears on paper to be rectangular if you don't know their little quirk.

Cushions are fancy cuts, and almost all fancies face up smaller per carat weight than rounds. Make sure you are looking at the optics of the stone, as cut quality is the most important factor - it's what makes it sparkle. A well cut stone will face up larger and whiter than a stone of equivalent weight that is not well cut.

 

[Slickk]

Re: Can you use surface area to compare size of two diamonds

I also use those formulas to calculate SA (surface area face up) being a math teacher. After you figure the surface areas, I would find the difference in percentage. In your example it's a 3.8% difference in face up surface area. So, not much of a difference really.

To find the percent difference subtract smaller SA from larger one then divide the difference by the smaller SA.
Change (difference) / original = percent change (multiply answer by 100 to turn decimal into percent)
Good luck!

 

[chrono]

Re: Can you use surface area to compare size of two diamonds

I also use the surface area calculation to know which one faces up bigger. As noted, make sure the LxW you are using is accurate.

 

[Niel]

Re: Can you use surface area to compare size of two diamonds

Hope this helps

 

[SirGuy]

Re: Can you use surface area to compare size of two diamonds

It's an interesting concept. I think it has merit for raw comparisons.

Something I think worth much consideration is the cut of the stone (not shape). Stones with varying qualities of light performance can exhibit more or less "edge to edge" brilliance. A stone might seem bigger based on how well its cut affects the perceived brilliance of the available surface area.

Also, depending on crown faceting with both rounds and other shapes, a stone might look a certain size from directly face up, and then change in perception when tilted off that axis (as in normal movement when worn).

 

[nukezero]

Re: Can you use surface area to compare size of two diamonds

Thank you everyone for your insight. Also, thank you Neil for that wonderful image comparison. It looks like it is not an accurate way of comparing size just by SA.

The ratio of the cushion is 1.0392 (LxW) so it is fairly square.

In order for me to get the best LxW size possible on the Cushion while staying in budget, I had to reduce the depth a bit so that more diamond mass goes towards increasing the LxW. The depth in this case is 56.7% with a table of 67%. A bit lower than a recommended 60-67% for cushion.

Also, I am using GIA certificates LxW for the numbers in my calculation. So not sure how super accurate that is. But that is the best I have. The only other "fair" equipment would be to use a precision digital micrometer.

 

[RADIANTMAN]

Re: Can you use surface area to compare size of two diamonds

The trouble with using surface area to figure out how big a cushion looks compared to a round is that the surface area of the cushion is extremely hard to calculate. LXW is the area of a rectangle, not a cushion. The rounding of the edges costs quite a bit of surface area vs. a rectangle, and just how much varies stone by stone depending on how rounded it is.

You can easily compare surface area of a round to a princess (LXW). With an emerald cut or radiant it's a little more difficult since you need to subtract the area lost due to the cut corners but you can get pretty close by subtracting 5-10% for the cut corners (depending on where they fall on the size spectrum).

A cushion requires a little bit more guesswork but in my experience cushions generally look a little smaller than radiants with the same LxW because all the edges are rounded, so LXW minus 10-15% will probably get you pretty close to the actual surface area on most cushions. That's just an educated guess, though.

 

[Gypsy]

Re: Can you use surface area to compare size of two diamonds

Thank you everyone for your insight. Also, thank you Neil for that wonderful image comparison. It looks like it is not an accurate way of comparing size just by SA.

The ratio of the cushion is 1.0392 (LxW) so it is fairly square.

In order for me to get the best LxW size possible on the Cushion while staying in budget, I had to reduce the depth a bit so that more diamond mass goes towards increasing the LxW. The depth in this case is 56.7% with a table of 67%. A bit lower than a recommended 60-67% for cushion.

Also, I am using GIA certificates LxW for the numbers in my calculation. So not sure how super accurate that is. But that is the best I have. The only other "fair" equipment would be to use a precision digital micrometer.

There are no recommended depths and tables for cushions in general. Anyone who tells you so is giving you inaccurate information. There are dozens of cushion facet patterns. Each facet pattern has different needs to maximize light performance. A cushion with 72% depth can be more beautiful than one with 63% depth 100% of the time, depending on the facet pattern and the angles.

You do not buy cushions from the numbers. If you do you will likely get an ugly stone.

Please read my posts in this thread: [URL='https://www.pricescope.com/community/threads/new-and-looking-for-help-choosing-cushion-cut-diamond-to-oz.205195/']https://www.pricescope.com/community/threads/new-and-looking-for-help-choosing-cushion-cut-diamond-to-oz.205195/[/URL] (all the way to the end).

Cushions are VERY different than rounds. Please make sure to read that post and follow all the links I've posted there. Once you've done that, we'll be happy to help pick out some stones for you.

 

[lioness88]

Re: Can you use surface area to compare size of two diamonds

Hope this helps

Love the graphic above!

Don’t mind my resurrecting this thread but where do the 4.49 and 4.23 measurements come from and how do they affect face up size?

Is it that they are the depth numbers and the deeper a diamond is the closer it shows up to the viewers eye, such that all things equal two diamonds w the same surface dimensions but different depths will appear different?

Also how do we explain the cushion looking smaller even tho its
surface area is bigger?

I get that a cushion might slope up more on top of the table but how does that negatively impact absolute surface area?

Thank you!!!! And wish I were less insane/analytical

 

[oldminer]

Engineers certainly can use surface area to determine which stone covers more square millimeters, but regular folks look at diamonds and can quite easily select the one they think "looks larger". You don't truly need to turn this into an engineering problem to be solved. Personal taste and personal selection trumps numbers for diamond selection.

More importantly, the way a diamond is cut can make a diamond look larger or smaller due to light leakage at the edges of the diamond, versus better cutting where the same exact size looks larger. That's the "Looks Like" feature added on to the HCA for round diamonds. The same applies to fancy shapes, but it is not easy to calculate. Again, your eyes are very good for this judgement call.

Shopping for a diamond that is incrementally larger in exact square mm within the girdle area is not the way to make a determination of quality or the thing to base a selection on.

 

[tyty333]

Love the graphic above!

Don’t mind my resurrecting this thread but where do the 4.49 and 4.23 measurements come from and how do they affect face up size?

They are the depth. They come from the GIA report (or whatever report the stone has). If you just put a carat weight
into the tool it will default the size (including depth) of the stone. The depths don't affect the tool because you are
given it the exact length and width. The depth does affect a stone though.

Is it that they are the depth numbers and the deeper a diamond is the closer it shows up to the viewers eye, such that all things equal two diamonds w the same surface dimensions but different depths will appear different?

Not exactly. A stone that is not well-cut and that does not have edge-to-edge brilliance will usually appear smaller than
a well-cut stone of the same surface area.

Also how do we explain the cushion looking smaller even tho its
surface area is bigger?

Its surface area is not bigger. The round has a 6% bigger surface area than the cushion.

I get that a cushion might slope up more on top of the table but how does that negatively impact absolute surface area?

Not sure how to answer this one. When we are talking about surface area its the physical size of the stone
(length X width).
Of course the bigger the slope the more carat weight in the crown which affects the overall size of the stone
(length X width). Just like if a stone is very deep the surface area of the stone will be smaller because more
of the carat weight is in the belly of the stone instead of the surface.

Thank you!!!! And wish I were less insane/analytical

Here is the link to that tool if you want to play with it. (if you don't have it already)

Diamond Database - Compare Diamond Shapes and Sizes

Diamond comparison platform where you can compare and evaluate actual diamond sizes on ring and finger. Explore diamond shapes, get buying advice.

www.diamdb.com

 

[lioness88]

Thank you! I totally agree that cut is the most important, just geeking out here and amazed by the wealth of knowledge on here. For some reason I thought the surface areas of both stones were about the same based on OPs calculations.

 

[Karl_K]

Surface area does not take into account 3d size appearance which is what you see with any tilt at all.
I had a thought exercise in developing a system for it but its super complex.
For example a princess cut or other flat topped stone would have a negative score. They look smaller with tilt.

 

[Garry H (Cut Nut)]

That cushion must be extremely shallow.
It has a spread about the same as an ideal cut 1.40ct round with a thin girdle.
that of course depnds on the curvature - because you need to deduct for the parts that are less than those dimensions.
I dont know if they are still doing it but I remember someone conned AGS into too measuring cushions from corner to corner. Very deceptive.