Diamond carat weight is the weight of the diamond measured in carats. One carat is divided into one hundred points: a diamond: a diamond of 75 points weighs 0.75 carats, or three fourths of a carat.
Carat weight is the most obvious factor in determining the value of a diamond. But two diamonds of equal carat weights can have very different prices, depending on their quality.
Since diamonds are measured by carat weight, and not size, two diamonds of the same weight could have different sizes. Why? If a diamond is cut to shallow (or flat) it will appear larger diameter, but shallow depth and appear larger. Likewise, if a diamond is cut to deep (or pointed) it will appear smaller as it has a smaller diameter.
This is why cut is so important. You want to make sure the diamond is cut to proper proportions. Not only does it make a large difference in the diamond's ability to reflect light, but a poorly cut diamond will not be proportionate in size to a diamond cut to proper spectifications.