# Diamond weights and actual size **URL:** \ **Category:** Factual Questions\ **Created:** [September 16, 2008, 1:28am UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776 "2008-09-16T01:28:35Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [September 16, 2008, 1:28am UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/1 "2008-09-16T01:28:35Z")
I’m trying to figure out how carat weight relates to the diamond’s actual size. Several charts I see online give the following: ```auto Carat Weight Daimeter 1.00 6.5 mm 0.85 6.2 mm 0.75 5.9 mm 0.65 5.6 mm 0.50 5.2 mm 0.40 4.8 mm 0.33 4.4 mm 0.25 4.1 mm 0.20 3.8 mm 0.15 3.4 mm 0.10 3.0 mm 0.07 2.7 mm 0.05 2.5 mm ``` But when shopping, I see [.25ct Blue Diamond Stud Earrings](http://www.amazon.com/White-Gold-0-25ct-Diamond-Earrings/dp/B000V5H1OC/ref=pd_rhf_p_t_3) > [@](#): > > …two full round cut 3.2mm blue diamonds in prong settings… [.50ct Blue Diamond Stud Earrings](http://www.amazon.com/White-Gold-0-50ct-Diamond-Earrings/dp/B000V5DVUU/ref=pd_rhf_p_t_2) > [@](#): > > …two full round cut 4.1mm blue diamonds in prong settings… [1.00ct Blue Diamond Stud Earrings](http://www.amazon.com/White-Gold-1-00ct-Diamond-Earrings/dp/B000V5H1O2/ref=pd_rhf_p_t_3) > [@](#): > > …two full round cut 5.2mm blue diamonds in prong settings… In short, the blurb doesn’t match any of the carat sizing charts. What am I missing? ---
**Author:** ![Green\_Bean](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/green_bean/32/1760_2.png) [@Green\_Bean](https://boards.straightdope.com/u/Green_Bean)\ **Post date:** [September 16, 2008, 1:40am UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/2 "2008-09-16T01:40:31Z")
> [@Patty\_O\_Furniture](#): > > In short, the blurb doesn’t match any of the carat sizing charts. What am I missing? The carat weight isn’t per earring. It’s per pair. This is how it’s usually done for regular diamond stud earrings. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [September 16, 2008, 1:51am UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/3 "2008-09-16T01:51:43Z")
There are also different cuts of diamonds. Two diamonds with the same diameter may have different weights if they are different shaped cuts. The most common is Brilliant, but there are others. > **[Diamond cut | Round brilliant](https://en.wikipedia.org/wiki/Diamond_cut#Round_brilliant)** > > Developed c. 1900, the round brilliant is the most popular cut given to diamond. It is usually the best choice in terms of saleability, insurability (due to its relatively "safe" shape), and desired optics. ---
**Author:** ![XJETGIRLX](https://avatars.discourse-cdn.com/v4/letter/x/9fc29f/32.png) [@XJETGIRLX](https://boards.straightdope.com/u/XJETGIRLX)\ **Post date:** [September 16, 2008, 3:17am UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/4 "2008-09-16T03:17:25Z")
> [@Patty\_O\_Furniture](#): > > What am I missing? The third dimension: depth. Diamonds even of the same cut, say round brilliant, can have different dimensions. Depending on how steep the cut, the stone may face up with a larger or smaller surface area but the same carat weight. Some stones have a much steeper cut than others. A shallow cut diamond will show more face and have a different look and level of brilliance to it than one cut at a deep angle. The depth of the crown and pavilion affect how the light bounces around in the stone. Check out this page for more information about how they are cut: [http://www.diamondsexplained.com/diamondcut.htm](http://www.diamondsexplained.com/diamondcut.htm) ---
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [September 16, 2008, 7:23am UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/5 "2008-09-16T07:23:53Z")
According to [Wiki](http://en.wikipedia.org/wiki/Diamond), diamonds have a densitiy of 3.5 - 3.53 g/cm³, which should make it quite easy to calculate the weight of stones of known dimensions. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [September 16, 2008, 11:30am UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/6 "2008-09-16T11:30:09Z")
It’s not even down to different cuts, it’s a simple matter of proportionality. I assume your thought process went something like this: " ‘.25ct Blue Diamond Stud Earrings …two full round cut 3.2mm blue diamonds in prong settings…?’ But two 3.2mm diamonds ought to add up to a 6.4mm diamond, which makes nearly 1 carat!" You need to remember that the weight goes up proportionally to the **volume** of the diamond, not to the diameter. Assuming (very simplistically) that the gems are spherical (volume = 4/3 π r[sup]3[/sup]) 3.2mm diameter = 1.6mm radius so volume = 17.2mm[sup]3[/sup]. So total volume for two diamonds = **34.4mm[sup]3[/sup]**. From the chart… a single 0.25ct diamond should have a diameter of 4.1mm. i.e. radius = 2.05mm so volume = **36.1mm[sup]3[/sup]** So you can see that two 3.2mm diamonds will have a weight very similar to one 4.1mm diamond, assuming they are roughly spherical. ---
**Author:** ![samclem](https://avatars.discourse-cdn.com/v4/letter/s/a9a28c/32.png) [@samclem](https://boards.straightdope.com/u/samclem)\ **Post date:** [September 16, 2008, 12:16pm UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/7 "2008-09-16T12:16:00Z")
**Green Bean** got the basic answer. The formula we use for a RBC(round brilliant cut) is diameter x diameter x depth x .0061 A Marquis, width x length x depth x .0058 Square cut (princess), width x width x depth x .0082 The stone can be a bit heavier when weighed if if has a thick girdle. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [September 16, 2008, 12:49pm UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/8 "2008-09-16T12:49:48Z")
> [@](#): > > You need to remember that the weight goes up proportionally to the volume of the diamond, not to the diameter. OK fair enough but that means the sizing charts provided by retailers are next to useless. Why provide a chart that indicates the stone’s diameter if there is a good chance it won’t be accurate? ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [September 16, 2008, 1:23pm UTC](https://boards.straightdope.com/t/diamond-weights-and-actual-size/463776/9 "2008-09-16T13:23:40Z")
> [@Patty\_O\_Furniture](#): > > OK fair enough but that means the sizing charts provided by retailers are next to useless. Why provide a chart that indicates the stone’s diameter if there is a good chance it won’t be accurate? I’m not sure what you mean by that? The sizes you quote in the OP demonstrate that the sizing chart _is_ pretty accurate. From the sizing chart: ```auto 0.15 3.4 mm 0.10 3.0 mm ``` You say that two 3.2mm diamonds made up a 0.25 carat pair of earrings, right? Well if each diamond is 0.125 carats, then you’d expect the diameter to be between 3.0 and 3.4mm, i.e. between the 0.10 and 0.15 carat sizes. Which is exactly as you do find. Similarly, a 0.5 carat pair will have 0.25 carat diamonds, and the chart says they should be 4.1mm diameter each – which is exactly what you found. Same with the 1.0 carat pair: each 0.5 carat diamond should have a diameter of 5.2mm, just as you found.