DIAMOND CARATDiamond carat weights is the measurement of how much a diamond weighs and 1 carat equals to 0.2 grams. So 5 carats are equal to 1 gram. The word carat has been borrowed from Greek "carob seed", a unit of weight.
Carat is a metric measurement and is divided by 100 points much like the interest rate to allow more precise measurement of weight. So often say 0.18 carat diamond is also expressed as 18 points. But diamonds larger than 1 carat are not expressed by points.
Larger sized diamonds are rarer than smaller sizes and diamond price per carat increases with its size. There is an approximate correlation with diamond sizes and weights.
The below chart is handy reference for indicative diamond sizes and weights.