Carat Diamonds are weighed in carats. One carat is the equivalent of 0.2 grams, or about 0.007 ounces. For practical reasons, carats are broken down in decimals called points. There are 100 points in one carat. The most common sizes in the jewelry trade range from 0.25 to 3.00 carat.
Larger stones are more rare, and therefore tend to be more valuable.
However, size often is the least important factor in determining a diamond's ultimate value. A poor cut, leaving extra material to save weight, will sacrifice the diamond's ability to reflect light. All the light gets reflected. 2. and 3. Due to the wrong cut, light is lost.