Carat is the traditional measuring unit of a diamond's weight (1 carat = 200 milligrams). A carat is divided into 100 "points," so the same diamond can be represented as weighing a carat and a half, 150 points or 1.50 carats. Moissanite stones are slightly lighter (less heavy) than diamonds. Therefore, a 6.5mm round diamond would weigh 1.0 ct while a 6.5mm moissanite would weigh 0.88 ct. However, the two stones would be the same size, 6.5mm in diameter.