1.5 Carat Diamonds are said to be 300 milligrams or 3/10 grams. Carat is the unit used for the weight of a diamond. The dimensions of a 1.5-carat diamond depend on its size, cut and clarity. Usually the diameter of 1.5 carat diamond is 7.3mm. Higher quality diamonds tend to be more expensive than lower quality diamonds (eg diamonds with higher colour, size and clarity are more valuable). Visit our online store for more information and to purchase 1.5-carat diamonds.