The weight of a diamond is expressed in carats (ct). One carat is divided into 100 "points" (for example: a diamond of 75 points weighs 0.75 carat). The word "carat" comes from the seeds of the carob tree, which having a constant weight is one of its characteristics so it was formerly used as the unit of measure for precious stones, until the system was standardized and one carat was conventionally set to 0.2 grams (a fifth of a gram).
Note that the weight/value ratio compared to the other features is not directly proportional. For example, if a 1 Carat gem is worth 1000, the value of a gem that is double its weight is worth much more than 2000 due to its greater rarity.