4 C's : cut, color, carat, and clarity   

Carat weight is the most common criteria inexperienced diamond buyers are aware of. The carat weight of a diamond is simply how much the diamond weighs. A carat is a unit of measurement equalling to 200 milligrams. Hence a 1.25 carat diamond will weigh 250 milligrams.

While the carat weight is certainly a factor when pricing out a diamond, it is by no means the end all. Many other factors must be considered including most importantly the quality of the diamonds cut. All factors (cut, color, clarity and carat weight) are used together to determine the value of a diamond. A shallow cut will lessen the value of the diamond due to the poor cut, but the diamond will have a larger diameter so that it 'appears' bigger. A deep cut diamond will have a smaller diameter than a ideal cut diamond, but will have a heavier carat weight since there is more volume (albiet wasted) to the stone. Assuming that we are comparing well cut diamonds to well cut diamonds, the price will increase dramatically as the carat size increases.
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These sizes shown below are approximate sizes only and are based on common diameters of well-cut diamonds. Actual size will depend on many factors including the depth of the pavilion which is not taken into account here.