Why do we use Diameter instead of Area to measure?

[Blueman33]

This first line was taken off another thread, but my response got me to thinking.

I hope an appraiser can explain why diameter is better than area.

~~~

Another thread said you need a %20 increase in diameter to be noticeable to the eye.


Do you mean 2mm?, or maybe 2%,

20% of a diameter change is very large.

a 1 carat at 6.5mm + 20% is

7.8 mm, which is a very large stone.

NO ONE SEEMS TO MENTION THIS, BUT DIAMETER IS VERY MISLEADING because it is one diminional. I think we should use AREA, not diameter.

In the above example the Diameter is 20% more, does that mean the diamond is 20% bigger? No. It''s 44% bigger. A = pi, r sq

Diamond One 6.5mm diameter. Diamond Two 7,8mm.

Diamond One -AREA
3.14 * 3.25 sq = 33.16

Diamond Two-AREA
3.14 * 3.9 sq =47.76

44% BIGGER, NOT 20%

I''m sure there''s some reason why AREA isn''t used, but it''s more clear to me.

 

[moremoremore]

I don't have an answer for ya...

Picture a princess and a round. They are both 8 mm in diameter...BUT, the circle still fits inside the square...which pokes out at the corners and has more surface area!!! I do like the regent diamond...but check out all those cut corners...still, the diameter is measured and its from the largest points...that doesn't consider how much surface area you lose..so you need to consider that when going for a particular size (e.t they are still so much more shallow than the deep princess cut)

 

[Lynn B]

Math challenged over here

...so I don't have too many answers for you, either... BUT I think I know about the 20% thing you mentioned.

I believe that means that in general, most people will notice a 20% increase in CARAT WEIGHT. For ex., a 1.20 diamond should look noticably larger to most people than a 1.00 diamond. To see the same type of difference from a 1.20 stone, you'd need to go up to a 1.44 or so. I have observed this myself when I was diamond shopping, and it seems fairly constant. (Of course we all know that certain stones are shallow-cut with a larger spread, etc., or deeper cut with a smaller spread... so this is only a general guideline...)

Lynn

 

[wonka27]

Which thread are you speaking of?

I have heard people talk about a change of about .20mm being around the point where they can see a slight difference in size. I know my g/f looked at .75 ct and 1.0 ct and could barely see the difference...which made me happy because I saved $2000 going with the smaller.

Why is area not used? I don't know. It sounds like you have a valid argument, but there are some things to consider. While measurement in mm is small, I think people can associate what the change is from one mm size to another. If you say 7mm and 8mm, people can comprehend what that measurement looks like. They can look on a ruler and see it. When you talk in area, it is hard to see a number and really equate it to anything. Furthermore, I believe mm measurements usually show a low and high diameter on reports, etc. It is good to see those number so you can determine some of the symetrical aspects. You don't want a diamond that has to much variation from low to high mm measured diameter.

Those are just my thoughts, but it is an interesting concept. I really boil it down to an understanding with one dimensional measurements.

 

[Garry H (Cut Nut)]

The stone on the right looks way smaller than the one on the left. (both are the same diameter).

A well cut round with the same surface area as a well cut princess looks way bigger.

 

[boonerings]

Yup, it's that whole pi thing. Most people can't relate to something as obscure as 3.14159265358979323... OK, I said most people.

To really get it right, we need to realize that the height of the stone also increases as the diameter gets bigger if we assume the same angles are used. The volume of the stone is what relates to carat weight, not diameter, or even area. I don't think even us techies want to be calculating that in our head! What is interesting is that the volume of a stone increases dramatically with a pretty small increase in diameter. Just adding a sheet of paper thickness to the diameter means that the entire outer surface of the stone gets larger by that much, and it all adds up to a lot more volume. That, coupled with the fact that larger stones are more rare in nature explains the hefty premium paid for stones even just a little bit larger.

 

[Blueman33]

The 'pi' thing. In school I thought all that was worthless.

Now I see that it tells me a 20% increase in diameter for the diamonds in question is really 44% bigger in surface area. area is sq. mm's.

So math did teach me something.

I would figure out the cu. mm's in a stone, but i'd have to go back to school (or get a computer program that would do it).

I can see .25mm difference at the 1 carat and below range (6.5mm and below) but sometimes have to look twice, i'm sure i could not see the difference in 10.00 and 10.25 mm.

 

[reena]

interesting. i too think that "area" is a useful measurement, especially for non-rounds because the varying L and W numbers make it really tough to figure out whether one stone will appear larger than another. when i was shopping for cushions i used an area calcuation to figure that out.

 

[Rhino]

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On 9/22/2004 7:55:49 PM Garry H (Cut Nut) wrote:

The stone on the right looks way smaller than the one on the left. (both are the same diameter).

A well cut round with the same surface area as a well cut princess looks way bigger.----------------

Excellent point and demonstration mate.

 

[Todd07]

I like this one. It seems pricing is primarily a $/carat with RAP adjustments for the grade and cut. This approach means deep stones or thick girdles increase price. If everyone also compared area, you could look at the $ per area (mm2) so spread becomes more important.

 

[Lynn B]

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On 9/22/2004 10:27:33 PM Ricardo07 wrote:


I like this one. It seems pricing is primarily a $/carat with RAP adjustments for the grade and cut. This approach means deep stones or thick girdles increase price. If everyone also compared area, you could look at the $ per area (mm2) so spread becomes more important.

----------------

Oh Lordy! Please!!! I would NEVER get it!!!

I think I must have day-dreamed through all the pi classes...

 

[wonka27]

Well, I think whatever anyone decides to do to help them find a diamond is excellent!

If your looking for more mathematical ways to work with diamonds, you may want to search for "spread index" here. Superidealist posted some pretty cool formulas that can help you determine what an "ideal" cut diamond's mm diameter should be at any ct weight and variations of the formula for determining other stuff. I won't go into too much detail...just look it up. It was fun to play with and I made a spreadsheet out of it.

 

[boonerings]

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On 9/22/2004 8:49:44 PM Blueman33 wrote: <hr size="1"
Now I see that it tells me a 20% increase in diameter for the diamonds in question is really 44% bigger in surface area. area is sq. mm's.

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As I said, area doesn't tell the whole picture. Here's a simplified example: If there was a cube 5 x 5 x 5mm, increasing it by a half mm per side would give 36sqmm area on the top rather than 25 sqmm. (a 44% increase) By adding that same thickness all the way around, which is essentially what happens when you increase diamond size, the volume goes from 5 x 5 x 5= 125mm cubed up to 6 x 6 x 6mm= 216mm cubed. (a 73% increase) That's a whole lot more difference than just the area shows. The extra 91mm cubed almost doubled the volume (thus the carat weight) of the cube by simply adding that measly 0.5mm per side. Carat weight is simply volume in cubic mm's times a density multiplier.

Real diamonds will be a lot harder to calculate than the cube because of their shape, but the increase in area needs to be remembered in the sides as well as the top. It adds up to be a much larger difference.

 

[Blueman33]

As I said, area doesn't tell the whole picture.

No, but it does tell a whole lot more than diameter.

Right now we use diameter, surface area is more straight forward. For your average person, when you say the diameter is twice as big, they don't immidately think, "ok a pie are square, 3.14 then half the diameter, then squar that number, then multiply....so it's really 4 times as big" they think, twice as big.

This is straight foward. "the face of the stone is 40% larger than the other stone, or the face of the stone is 3 times larger, etc.". The face is what we see. The diameter is misleading to the average person.

Volume is more complicated. We don't see volume the way we see surface area. If it didn't affect performance we'd all like shallow diamonds so they have a big face!

 

[denverappraiser]

Blueman,

The labs use dimensions because they are reporting direct measurements. Consumers use dimensions because that’s what appears on the lab reports. Area can be calculated from the dimensions but not the reverse.

Neil Beaty, GG ISA
Independent Appraisals in Denver

 

[Blueman33]

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On 9/23/2004 8:53:09 AM crankydave wrote:

Blueman33,

Perhaps I'm the one that's confused but it seems you're equating a .20 mm difference to mean a 20% difference which is not the case. If a stone is 6.0 mm in diameter a .20 mm difference would be 6.20 mm stone or a 3.4 % difference in diameter. I'm able to tell very slight differences in smaller stones but the larger the get the larger differnce needs to be.

I'd say, in general, when the difference in diameter is 10% or more you'll be able to tell the difference. For example from a 2.0 mm stone to a 2.2 mm stone or a 6.0 mm stone to a 6.6 mm stone and so on.

Dave


No, i'm just saying that 'diameter is misleading because you don't see diameter, you see area. So if you want to talk mm, then .2 mm is 3.33% change from 6.0 to 6.2, it is a 2.5% change from 8 to 8.2, and so on. BUT it's not a 3.33% and 2.5% change, it's more. Because a change in diameter is misleading and doesn't take into account the increase of, say 3.33% is around the whole circumfrance, not just a line.

 

[Blueman33]

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On 9/23/2004 9:09:30 AM denverappraiser wrote:

Blueman,

The labs use dimensions because they are reporting direct measurements. Consumers use dimensions because that’s what appears on the lab reports. Area can be calculated from the dimensions but not the reverse.

Neil Beaty, GG ISA
Independent Appraisals in Denver

----------------

Any formula with three variables, two of which are known, will calculate the third.

Diminsions of a round diamond can be obtained if the area is known, since we know what 'pi'is.

A= pi r sq.

Area, which we have, divided by 3.14 equals the radius squared. You take the sq root of the number obtained by Area/pi then double it, since radius is half of diameter.

I honestly think that for round diamonds, there must be a history of using this measurement which is not straight forward. I see no logic behind it, apparently that's just the way it's done. I have seen no compelling argument as to why area isn't a better measure on a RB than diameter.

 

[quaeritur]

Personally, I prefer the diameter measurements because I can tell how out-of-round a diamond is, which does influence symmetry to some degree. With just an area measurement, while I could come up with some average diameter, it wouldn't tell me if I was looking at a round, ovalish or squoval diamond...

 

[denverappraiser]

Blueman,

Round diamonds are often not particularly round and as Q points out, this can be important information. Your formula assumes that the perimeter is a perfect circle.

Neil Beaty, GG ISA
Independent Appraisals in Denver

 

[Blueman33]

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On 9/23/2004 3:19:15 PM denverappraiser wrote:

Blueman,

Round diamonds are often not particularly round and as Q points out, this can be important information. Your formula assumes that the perimeter is a perfect circle.

Neil Beaty, GG ISA
Independent Appraisals in Denver----------------

Round diamonds are not particularly round, but are close.

Diameter doesn't do a good job of showing the size of the diamond.

You argument is that area doesn't show the roundness or out of round. Ok, that's true. But it is a separate argument. I NEVER said the measurements were mutually exclusive. I never advocated not disclosing the roundness or out of roundness.

Diameter is a misleading measurement for judging the size of a diamond.

When the size of a diamond on paper stated in diameter is around 100% off the actual size of the face of the diamond, that is very noticeable with the naked eye.

Within the scope of an 'out of round diamond' such as 6.90-6.98 x 4.17, you can't see it with the naked eye.

Area is a more straight forward measurement for the size of a diamont; it doens't take in 'out of round' which cannot be seen with the naked eye, but is already stated anyway in the diminsions.

BTW, the diamond above as out of round as it is, was GIA EX for sym.

 

[Blueman33]

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On 9/23/2004 4:19:37 PM crankydave wrote:

Blueman33,

Okay, let me put into a craftsmans/designers perspective. Area tells me nothing. As you pointed out, area will always be a constant based upon the precise dimentions. However, you cannot extrapolate the dimentions from the area. When selecting the proper bur, head, crown, setting, mounting etc. the diameter and the shape are imperative. I need to know whether the particular stone is more oval than round or more rectangular than square. Quite logical.

Dave

Edit to add:

Because a change in diameter is misleading and doesn't take into account the increase of, say 3.33% is around the whole circumfrance, not just a line.

Not if you're designing a ring, setting a diamond, making a bezel or a basket etc. It's quite straight forward.
----------------

You will see the post is for AVERAGE people trying to determine the size of the ring. Your comment is not on this premise.

If area tells you 'nothing' then I have a lot of land to sell you. (see, i can take the argument off premise as well) : ) now seriously....

I never said ring designers could design a ring and set a diamond better if they used area, i said area is a better measurement for the average person than diameter for judging the size of a stone.

I do not set jewelry, so I will take you at your word and agree with you.

There are now two posts that say you cannot extrapolate the diminsions from the area.

It is a mathematical FACT, not an opinion, that with a circle (round diamond) and the area known you can extrapolate the diameter, just as with a circle (round diamond) with the diameter known you can extrapolate the Area.

Does it assume a perfect circle? YES, just as most AVERAGE people assume a 'round' stone is a circular one.

To the extent the a 'round' diamond isn't round, it is a very very tiny bit off (especially since you would use the average diameter), BUT it isn't anywhere close to as much off as a measuremnet which is 100% or more off like when the average person uses diameter to rate the size of the stone.

Imagine a big diamond and a bigger one.

A DIME 18mm
A QUARTER 24mm

Look at those coins side by side. One is much bigger. Set the dime on top of the quarter meeting on one edge, and you will see that by diameter the Quarter is only 33% "bigger" than the Dime, but the diameter only takes into account one diminsion and therefore is misleading to the average person. It's really 77% bigger.

 

[Blueman33]

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On 9/24/2004 1:15:39 PM crankydave wrote:

You will see the post is for AVERAGE people trying to determine the size of the ring. Your comment is not on this premise.

If you are referring to my second post, you are absolutely right.

This was your original post....

This first line was taken off another thread, but my response got me to thinking.

I hope an appraiser can explain why diameter is better than area.


yes, and not one has, so i'm hoping for consumers this measurement will start being used. perhaps PS or a vendor will start using it.
~~~



Another thread said you need a %20 increase in diameter to be noticeable to the eye.

TAKEN OFF ANOTHER THREAD, but not MY post. I disagreed with it.

This is how I answered your post taking into account the AVERAGE person...

Blueman33,

Perhaps I'm the one that's confused but it seems you're equating a .20 mm difference to mean a 20% difference which is not the case. If a stone is 6.0 mm in diameter a .20 mm difference would be 6.20 mm stone or a 3.4 % difference in diameter. I'm able to tell very slight differences in smaller stones but the larger the get the larger differnce needs to be.

As I said, NO, I am not equating any of that. Clearly a .20mm difference would only be 20% in the case of 1.0 to 1.2.


If an AVERAGE person was looking for a pair of stones to sit along side of a center stone I'm quite certain they would want those 2 side stones to match each other FACING UP. IF the AVERAGE person assumes (a mistake) the two stones match based upon area and not diameter(s) they could very well be disappointed. All other factors being equal, diameter and not area will determine how well these stones match to the AVERAGE person FACING UP.

No, this is not an opinion, two round stones of the same area BY MATHEMATICAL FACT have EXACTLY the same diameter, whether you are AVERAGE Joe or a Rocket Scientist. If you are talking about round stones that are not round, then they do not technically EVEN HAVE a diameter. A diameter by defintion is for a circle. There is no diameter for an oval, square or triangle.

The only way this argument makes sense if you are confusing area of a circle, the face of the stone, with volume of the stone.

To the extent the a 'round' diamond isn't round, it is a very very tiny bit off (especially since you would use the average diameter),

No. The variance in diameter is noted on certs and most appraisals. That tiny bit can make a stone look 'out of round' to the AVERAGE person.

You are the only one who has ever said that. If it is noticeable with the naked eye, then it's not a round stone. One must assume that the confines of a 'round' stone and very very small, and that an oblong or oval one is just that...oblong or oval. if the grading agencies call stones that are significantly 'out of round' to be 'round' then they are incorrectly using the term 'round' and incorrectly using 'diameter' to discribe other than a circle.

I will clarify

For the Average person, diameter as a measure of the size of a round stone is misleading since it is one diminsional and area is a better indicator. THIS IS ONLY FOR ROUND DIAMONDS, NOT OTHER SHAPES.

Edit to add:

just as with a circle (round diamond) with the diameter known you can extrapolate the Area.

Extrapolate away! Noone wants anyone to feel misled.

Anyone can! High School Math. A= pi r sq.

Hopefully a vendor will pick it up and start adding the size of the face of the round stone to tell people actually how big it is, so they will have a clearer view of size relationships when looking at a round stone or comparing two stones. If it catches on other vendors will follow. Or perhaps an influential person in the industry can make some headway to help the consumer!


Dave



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[Garry H (Cut Nut)]

All this seems great, but you have ignored my post about light return.

If you compare a princess cut to a round diamond with the same surface area, the round will appear to be larger.

In addition there is another type of 'spread' that I mentioned some years ago - Vertical spread. An AGS 0 diamond with a 47% table and 34 degree crown angle has a 17.9% crown height and when seen from the side would appear to be larger. The diameter difference would only be less than 0.1mm.

So Dave, I think your system is only slightly less advantagous than diameter as measure.

 

[aljdewey]

A perspective from a woman who actually wears a diamond.....:

Most women don't give a rat's behind about the "area" of their diamond.

What does a woman care about? How it looks on her finger, which generally means "how much of my finger width does the diamond span?". That's typically left-to-right....or diameter.

 

[fire&ice]

Why - because it's the measurement used in the trade. and can be quantified from looking at the cert.

I'm not one to change the way diamonds have been viewed for centuries for proper math.

 

[quaeritur]

----------------
On 9/27/2004 11:25:21 AM aljdewey wrote:

A perspective from a woman who actually wears a diamond.....:

Most women don't give a rat's behind about the 'area' of their diamond.

What does a woman care about? How it looks on her finger, which generally means 'how much of my finger width does the diamond span?'. That's typically left-to-right....or diameter.----------------

I was sitting here with my brain hurting from this thread, trying to figure out why else I like the diameter measurement -aljdewey just nailed it, thank you!!

Also, I personally can't visualize area. I can visualize 7mm. 38.484510006474967171167381445174mm^2 means diddly squat to me. It would ONLY be helpful if used comparatively. But I can already do that with diameter, which is helpful on its own. So I continue to believe the diameter is the more useful measurement.

Maybe a CAD drawing would be more helpful on the cert... Actually show the diamond. What I'd love to see, and would be more helpful yet, is an IS image on the cert!!!

[edited to fix my math

]

 

[Garry H (Cut Nut)]

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On 9/27/2004 11:25:21 AM aljdewey wrote:

A perspective from a woman who actually wears a diamond.....:

Most women don't give a rat's behind about the 'area' of their diamond.

What does a woman care about? How it looks on her finger, which generally means 'how much of my finger width does the diamond span?'. That's typically left-to-right....or diameter.----------------

Hey Aldjeway, I do not give a rat's about the area behind either.......I am an upfront man myself

We should use cup size, not meauserments in inches?

 

[JohnQuixote]

...Giving H"C"A a whole new twist!

 

[dimonbob]

Back to the original question.

The diamond industry uses diameter rather than area for the same reason that the United States uses inches and feet rather than the metric system.

Once you have started using a system and everybody uses it, it is very difficult to change. The powers that be have been trying to change the United States over to the metric system for decades.

Change is SLOW!

 

[Blueman33]

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On 9/27/2004 1:52:28 AM Garry H (Cut Nut) wrote:

All this seems great, but you have ignored my post about light return.

If you compare a princess cut to a round diamond with the same surface area, the round will appear to be larger.

I wasn't advocating using area to compare diamonds of different shapes. A princess cut doesn't have a diameter. I am merely advocating that for round diamonds, area is more straight forward than diameter as diameter is one diminsional and are is two diminsinal.