Carat

The unit of measurement of a diamond is called carat weight or carat in short. 1 carat (ct) is equal to 200 milligrams (mg) in weight wherein each cart is divided into 100 points. Similar to currency, a 0.50 ct. is equal to 50 points and a 0.25 ct. is equal to 25 points.A related term, Total Carat Weight (TCW), this is the total carat weight of several pieces of diamonds. For example, a diamond engagement ring with a center stone of 1.00 ct., side stones of 0.50 ct. and accents stones of 0.32 has a Total Carat Weight of 1.82 TCW

[NOTE: Don't confuse "carat weight" with "karat," the method of determining the purity of gold.]
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.It is therefore possible to have a diamond of a lower carat weight, but higher cut grade, that appears larger than a diamond with a larger carat weight, but poor cut.

What Size Diamond Should I Buy?
 

• First, determine your budget. One general rule of thumb when buying a diamond engagement ring is "two months salary." This is just a guideline, it's not carved in stone and your first consideration should be what you can comfortably afford -- not what the diamond industry or a jeweler tells you (they are not unbiased in this matter).
  
• Deciding on carat size is really about striking a balance between size and quality. If she prefers larger jewelry items, and you are working within a budget, you can still find a larger diamond of excellent quality gem by selecting one which is graded slightly lower in terms of color and clarity. Remember that slender fingers make small diamonds look bigger. If she has small fingers, a 1-carat diamond will look proportionately large -- and an even larger stone may appear stunningly big!
  
• Think about what sort of setting will hold the diamond. You'll have to be sure that the setting you choose is made to fit the carat weight of your diamond.