A unit of weight for a diamond. 1 carat is equivalent to 200 milligrams, or 1/5th of a gram. It takes 141.75 carats to make 1 ounce, and 2,268 carats to make 1 pound.
The word “carat” today is thought by some to be derived from the ancient word for “carob.” In ancient India, the carob bean was used for measuring the weight of gems, because of it’s rare property that every seed weighs precisely the same.
Wholesale prices of diamonds are listed per carat in the various pricing charts (such as the well-known Rapaport Diamond Report) to represent the varying costs, since larger diamonds cost much more than smaller stones, due to rarity of larger stones. For instance, a 1/2-carat (0.50 ct) round brilliant diamond of G Color Grade / VS2 Clarity Grade might sell for $3,600 per carat ($1,800 for the half carat stone), while a 1.00-carat stone of the same quality would cost $6,300 per carat ($6,300 for the one carat stone) because it is more rare.
Do not confuse Carat Weight with Carat Total Weight (ctw), which is very different.
Here is the GIA intoductory video about Carat Weight from their website:
Here are some carat sizes (approximate!). For best results, please make sure your screen is set for the size that makes the dime close to actual size.
Here is another graphic that attempts the same thing ::
Obviously, the only way to be sure you are comparing accurately, is to see them in person. Computer screens vary too much to do this online.