The carat is the traditional unit of measurement for a diamond’s weight. Moissanite is not measured in carats because it weighs approximately 10 percent less than diamond. For example, a 6.5mm round diamond would weigh 1.00ct, while a 6.5mm round moissanite would weigh 0.88ct. The two stones would be the same size – 6.5mm in diameter. View the charts below to compare mm/carat conversions for multiple stone shapes & sizes.