Diamonds are sold by the carat (shown as ct.), which is actually a unit of weight, though most think of a carat in terms of size.
One carat is equal to exactly 0.2 grams. Carat weight is unrelated to the similar sounding karat, which refers to gold's purity.
Two diamonds of equal carat weight can have significantly different costs because of the other factors: cut, color and clarity.
As a diamond increases in size and carat weight, the price will increase at an increasing rate because larger diamonds are more rare and thus more valuable.
To better understand diamond’s size, we recommend to consider carat weight with two other characteristics:
1.) the distance across the top of the diamond measured in millimeters, and
2.) the diamond's cut grade.
It is important to measure the distance across the top of the diamond as this is how we view a stone when set into a ring. A diamond's cut grade should also be considered because, as we noted in the cut grade section, when a diamond is cut with the proper proportions, the maximum amount of light (or sparkle) is returned out of the top of the diamond. Thus, when a diamond is well cut, the light reflected out of the top makes it appear larger. In addition, much of the weight of a poorly cut diamond, for example, may be "hidden" in the base of the diamond, making the diamond appear smaller than its carat weight would imply.
Once you've selected your cut, colour, and clarity grade, it's easy to determine the carat weight of a diamond that will fit within your budget.
When choosing the carat weight of the diamond, consider these:
1.) The size of the finger. The smaller the finger, the larger the diamond will appear.
2.) The size of your setting. Not all settings will fit all diamond carats or shapes. If you have already selected a setting, start by checking the diamond specifications of your ring.
3.) Your budget. If a large carat weight is important to you, yet you're working within a strict budget, consider a diamond with a good cut, SI1-SI2 clarity, and an I or J colour grade.
Notice: Diamond prices jump at the full- and half-carat weights. Diamonds just below these weights cost significantly less, and, because carat weight is distributed across the entirety of the diamond, small size differences are almost impossible to detect.