Newbie: MM to carat conversion - round versus princess?

[Sparkly Stuff]

Forgive me if this is a stupid question, but I''m a newbie to the diamond world. I was looking at the MM to Carat Conversion Chart posted on this forum and I noticed that, for example, .20 round = 3.8, and a .20 princess = 3.25 x 3.25. Does this mean that a round stone of the same carat weight as a princess will generally appear larger (3.8 compared with a square of 3.25 x 3.25?). Maybe there are other factors involved - cut proportions, for example? Thanks to anyone who can enlighten a newbie.

 

[chialea]

If you look at Dave Atlas' cut charts, you'll see (if I've interpreted correctly) that a well-cut princess stone is deeper than a well-cut round. Certainly the ones looked at around here seem to follow that rule. So you're hiding some weight down there. However, you're also looking at, for an eqivalent mm measurement (assuming square) pi*(# mm/2)^2 area for the round, and (# mm)^2 for the princess, so it's not exactly a fair comparison -- more surface area may look bigger than just bigger diameter.

If you're looking at equivalent surface, then the round must be 1.13 times as wide to have the same surface area. (chopped some signifigant digits)

 

[cflutist]

Face up area of the princess is 3.25x3.25 = 10.56 mm
Face up area of the round is 3.1416*1.9*1.9 = 11.34 mm
So the round will look larger face up.

 

[chialea]

... that's if you're going for the same carat weight. I believe princess stones tend to be somewhat less expensive by the carat, so that's where the diameter propotion rule comes in -- so you can compare apples to apples.

 

[Garry H (Cut Nut)]

I do not have an example picture of round vs princess, but in this case of 2 round same diameter CZ's - one ideal and one bad cut - you can see that the well cut stone appears larger.

Even if a round diamond has the same surface area as a princess cut, the round will L@@k larger.

 

[moremoremore]

There was a post on Diamond Talk a long time ago about this...someone came up with a number that you need to reach to have the round look just like the princess!!! Can't find it!!!

Picture a circle in a square....They both have the same diameter/mm measurements...BUT the circle fits INTO the square!!!! As stated above, the princess of the same mm measurement will look larger than the round b/c it has more surface area!!! You will also have to have a larger princess in terms of carat weight to get to the same mm as the round of the lesser carat weight...but also as stated above, the princess is cheaper per carat!!! God Bless Pointy Corners!!!

 

[moremoremore]

Gary- I apply that same principle to dressing...I'm so "thin" in all this black!!! (NOT!)

 

[moremoremore]

I'll give you the thread...this is from DBOF...
1 carat princess, next to 1 carat round...

 

[moremoremore]

Here is the addition of a 1.5 princess...

They are 5.5 mm, 6.5 mm, 6.5 mm.... but I think this is cheating b/c the first stone has a 65 depth or something!!!!! The second princess is 71+ depth I think...here is the thread...

http://www.diamondtalk.com/forums/showthread.php?threadid=13657

 

[moremoremore]

On dirtcheap...a 1 carat Round F vs2 is about 6500-7000?
A princess 4400-4800?
A 1.25 is about 6800?

 

[headlight]

Garry H/Cut Nut -- Sorry to ask such a stupid question, but which stone in the photo you posted is the well-cut stone? Is it the one on the left -- and the one on the right shows light leakage??? Except that to my eyes, the one on the right looks larger.

 

[Sparkly Stuff]

Wow, a picture really is worth a thousand words - !! Going strictly by weight, the round definitely appears larger, but going by mm, smaller. I guess this explains why rounds are more expensive per carat? Interesting - I'm getting a real education. PS, to my eyes the CZ on the right is the better cut and larger looking one. If not true, I stand corrected.

 

[Garry H (Cut Nut)]

Just goes to show how useless photo's are

The ideal cut signity CZ is on the left side.
These are within .05mm of the same diameter - it is possible that the bad stone is a little wider in diameter.
But the bad stone on the right weighs about 15% more.

Part of the problem is that the good CZ's has photographed very white with no dark contrast.

You can chaeck our http://diamonds.pricescope.com/carat.asp example too.

 

[valeria101]

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On 7/21/2004 7:25:11 PM Sparkly Stuff wrote:



Wow, a picture really is worth a thousand words - !! Going strictly by weight, the round definitely appears larger, but going by mm, smaller.

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Hm... of course a square with the sides as large as the diameter of a circle will have lots of extra area than the circle (27% exactly).

Actualy, that circle will just fit inside the square leaving those four tips extras. This is not fair game on the poor RBC.

To get a square with the same area as a circle, you need the side to be 88.6% of the circle's diameter exactly! This doesn't tuck the tips of the square inside the circle completely, but almost. [ BTW. To get the square completely inside the circle (and blatantly smaller) you need the side to be 70.7% of the round's diameter]

For example, taking 6.5mm as the diameter of a 1ct round, you'd need a princess cut with a side of 5.75mm. this means 1-1.15cts from 65% depth to 75% depth. At this weight range, it would be the thickness of the girdle and crown height that would account for those extra 10 points required. So the numbers are not "decisive" and it's easier to compare diamonds than make up some 0.2 theory

For what this is worth...

As far as my sight goes, two shapes of the same area tend to look about the same size, most of the times, especially if there is no blatant difference in any direction. For example, a looong marquise with the same area as a round may appear larger because of the impressive length, but a square would not get the benefit of the doubt. Accordingly, if you want the largest diamond for a given weight, that is going to be a shallow, thin stone: marquizes, ovals and pears qualify for the honor. Another cut would be long ECs or baguettes, but I might well be the only one left to like large baguette cuts - wattery as they often are...

 

[valeria101]

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On 7/22/2004 6:52:33 AM Garry H (Cut Nut) wrote:



Just goes to show how useless photo's are

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I was thinking of that picture quite a few times. This must be the second time it gets cited and the second time that it fails to tell the tale... if I remember right.

I know that whiteness means "light" and "sparkle" but the immage is so exceedingly bi-dimensional, that it takes a good leap of faith to associate it with a living, breathing diamond... And the complicated reflection inside that lousy cut doesn't fail to look pleasantly intriguing even to my H&A seeking eyes!

On the other hand, the only picture of a 3D "ideal versus dubious" diamond around here is the tiny one in the tutorial at Preciousmetals. The thing is about as big as my thumb and should be the flag of PS credo

Is there a representative version (large, high definition...) anywhere? The setup of the three stones (ideal, rough and dubious) seems well worth more photographic art - to me at least...

Just the respective 0.2, of course

 

[Garry H (Cut Nut)]

Ana if the round returns a lot more light than the same surface area square, then the round will look larger.

I have done this as an experiment / survey. Princess cuts need to have a considerably larger surface area to look the same size in carat weight.

 

[valeria101]

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On 7/22/2004 7:36:33 AM Garry H (Cut Nut) wrote:

Ana if the round returns a lot more light than the same surface area square, then the round will look larger.

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Sure that - same as the picture of the two CZs show for rounds. But does it show? The result is obvious to me, the picture is less.

Anyway, once could just wish that diamonds could be better uploaded in a picture. And it is just my 0.2 along the lines of this thread, of course. I found it funny that a round and a princess would be compared for size using GIA measurements (diameter and side length) againt any logic on one of the threads above...