I’m trying to figure out how carat weight relates to the diamond’s actual size. Several charts I see online give the following:
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
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.
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
According to Wiki, 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.
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.
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:
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.