In 1907 the metric carat for gemstones and metals was adopted into use by the Conference on Weights and Measures. The carat is equal to 200 milligrams or 0.2 grams. The origin of the word "carat" comes from the carob seed. Gemstones were once weighed on balance scales, and gem dealers used carob seeds as counterweights, because of their uniform shape, size and roughly equivalent mass. Carat weight was standardized to .20gm in the early 20th century. When speaking in diamond terminology - a carat is made up of 100 points, so 25 point diamond would be referring to a quarter carat size diamond.
This size of a diamond is the quickest assumption of value prior to a consumer learning more about diamonds. Different customers choose size over quality, while others would rather have a smaller diamond with a better balance of cut, clarity and color.
Below we have a millimeter guide that is estimated upon ideal diamond carat weight proportions.