Out of the 4 C's Carat is the easiest to determine. Carat is actually a measurement of weight rather than size. However it should be relative to size as well. One carat is equal to 0.20 grams (weight). In the diamond industry "1.00 carat" is divided into 100"points", so for example a 70 "points" diamond weighs 0.70 carats

0.30            0.50            0.75            1.00            1.25           1.50             2.00