The weights of diamonds and most other gems are expressed in metric carats. One metric carat equals 0.200 grams (200 milligrams or 1/5 gram). A carat is divided into 100 equal units called points. According to US Federal Trade Commission rulings, all weight representations are subject to a 1/200 carat (one-half point) tolerance. If other factors are equal the more a stone weighs, the more valuable it will be.