Everyone knows more Carat weight means more cost. But the surprise is how much more BIG diamonds cost; if you double the weight then the cost is about 4 times more.

You know that Carat weight equals size? Wrong! The two diamonds in the picture on the right are the same size or diameter; each stone could be cut from the rough diamond. But because of its heavier ‘magic’ one carat weight the dull stone sells for more than the sparkly one.

It is economically tempting for a cutter to leave a little more weight on the crown, pavilion (the top and the bottom) and the girdle as it can push the stone to the next ‘magic weight'. On Pricescope you will learn about tools like HCA, the Ideal-Scope and the ASET that can help you choose wisely. You can post questions about specific stones on the Rocky Talky Forum.

The magic weights are 1/2ct, 3/4ct, 90 points, 1ct, 1.5ct, 2ct etc. ‘Under-sizes' are diamonds that weigh just below a magic weight; they can be a bargain, but there are a lot less to choose from.

The girdle or edge thickness is also important. If a diamond has a very thin girdle, the stone can have a better spread, but it can chip more easily. Medium to slightly thick girdles are safest, but thicker girdles add extra weight or reduce the spread.

To get an idea about the range of spreads for different weights, search the Pricescope listed vendors in-house diamonds and look for the value next to the price on the far right. Say a 1ct Princess cut is listed at -10%, that means it has the same spread as 0.90ct textbook round diamond.

International weight rounding is not the same as what you learned in school. Diamond weights are rounded up from the third decimal point only when it is a nine.
0.995ct = 0.99ct
0.998ct = 0.99ct
0.999ct = 1.00ct

Set diamonds cannot be weighed so carat weights are estimated by measuring the outside dimensions of the stone.This is common for an appraisal, which is a report that states the value of finished article, but it would be wrong for a grading report (or ‘cert’).Such an appraisal should state ‘estimated’ or ‘approximate’ weight and an error about 5% is common.(The color will sometimes be given a split grade of say H/I indicating the grade is also an in mount estimate).

Diamond weight in set jewelry is often ‘total carat weight’ or ctw; 2.00ctw of center plus side stones is very different from 2.00cts. It is legal in some countries, e.g. the USA, to describe a batch of goods as '1 carat rings' even though the diamonds range from 0.95ct to 1.10ct. In theory the average should be at least 1.00ct. 'Buyer beware'.