The bigger the diamond, the more it costs and has a higher cost per carat as well. Diamonds are measure by their weight, Carat (abbreviated ct.) is the standard unit of weight measurement. One carat refers to 0.200 grams or approximately 0.007 ounces. To give you a relative point of reference, a very small plastic paper clip or a carob seed from carob tree, weighs about a carat.
When talking of diamond size, we generally use the word points. A one-carat is a 100 points; so a 50 point stone will be a half a carat and 75 points refers to three quarters of a full carat weight. On the higher side, a 100 point is referred to as a carat and up to two decimal places for beyond full cartage like 1.09 is a carat and 9 points.
Its diamond industry practice to weigh diamonds to a thousandth of a carat, then round the weights to a hundredth of a carat. You round the weight up only if the number in the thousandths place is 5 or more. For example, you'd round a diamond that weighs 1.764ct. to 1.76, but one that weights 1.765ct. to 1.77.