Using carats when comparing modern round diamonds is better because they are relatively uniform. All other stones should be compared using both: their weight and measurements. Millimeters are helpful to compare fancy cut diamonds or colored stones because their size does not directly correspond to carat weight. Stones with identical weight can vary significantly in size. Some “spready” stones can be twice as big as deep stones of the same weight. Elongated stones can have different length-to-width ratios, which is an important consideration. Colored stones have a different density than a diamond. Comparing sapphires to a diamond by weight is fruitless because sapphire density is much higher. A sapphire is smaller than a diamond of the same weight.