The weight of a diamond carat is different from the weight of a cubic zirconia due to differences in density. A cubic zirconia is approximately 75 percent heavier than a diamond. Also, cubic zirconias are sized in millimeters and use carat weight for comparison to diamonds. A one-carat diamond is comparable to a 6.5 mm cubic zirconia, but the cubic zirconia weighs 1.75 carats. When sizing a cubic zirconia, jeweler's equipment is used and basic knowledge of loose stones is needed. How to Measure Cubic Zirconia Sure, diamonds are forever, but they're also very expensive.