Diamonds are measured in Carat Weight. One carat weighs 200 milligrams. If a diamond is referred to as four grains, this also means that it is a one carat diamond. The word Carat comes from the word carob. A carob is a bean that grows on a tree in the Mediterranean. In times past, if a diamond weighed the same as a carob bean, it was one carob, or one carat.
However, in the far east, where Carob trees do not grow, rice was used to measure the weight of a diamond. If a diamond weighed as much as four grains of rice, it was four grains - or one carat as we know it to be now. The majority of diamond purchases are for diamonds that are 1/3 of a carat.
Beware when shopping for diamonds that are already set or mounted. If more than one diamond is used in the piece, the tag on the jewelry will give the CTW or Carat Total Weight - it does not tell you the carat weight of each stone in the piece. You need to ask the jeweler for the total carat weight of the largest diamond in the piece to truly understand what you are buying.