Diamond and most other gem weights 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.
Diameter and Weight:
Estimating Melee Weights By Diameter: