Most people think of a diamonds carat as size. The diamond carat is more accurately defined as a measure of weight. The carat is a standard unit of measure that defines the weight of a diamond, which is equivalent to 200 milligrams. Carat sizes are also expressed as "points", where a one-carat diamond equals 100 points, a one-half carat diamond being 50 points, a three-quarter carat diamond being 75 points. The GIA certified diamond report specifies the carat weight to two decimal places (ex. 2.25 carats).

Diamonds of the same carat weight can optically look different sizes. Since a carat is a unit of measure and not size, two diamonds of the same carat weight may appear to be different sizes depending on how the diamond is cut. Some diamonds will have extra weight on the bottom part - or pavilion - of the stone, and therefore appear smaller. A shallow cut diamond will have a larger diameter and will look larger from the top than a correctly proportioned diamond. Also diamonds with a larger table size will look optically larger when viewed from the top. However the proportions of the diamond are vital to the fire and brilliance of the diamond. The cut grades should be checked for any diamond certificate to ensure proper sparkle and brilliance. On a GIA diamond certificate the best grade is an "excellent" proportion.

Larger diamonds are much more rare than smaller ones. Hundreds of tons of rock and ore must be mined to uncover a single one-carat gem quality diamond. A 1 carat diamond will be much more expensive than a half carat, much more expensive than double. This is due to the fact that there are fewer 1-carat diamonds available.

Buying the largest diamond without consideration of the other value factors will likely end up with a stone of such inferior quality that it will simply look glasslike when your partner compares it to her best friend’s diamond. Side by side a diamonds’ cut is what gives the diamond FIRE is crucial brilliance.

Diamond wholesalers have long known that you should not buy your diamond alone based on carat weight. Other factors will play into the value and beauty of the diamond. The difference between your GIA excellent cut diamonds vs. inferior diamonds is the difference between a WOW diamond and a low quality glass-like diamond.