A 2.5 carat diamond weighs around 2.5 carats, which is equal to approximately 500 milligrams or 1/2 gram. A 2.5 carat diamond's size is determined by several factors, including the diamond's shape and cut, as well as its clarity and color. A 2.5 carat diamond will be extremely enormous in size, with a diameter ranging from 9.0 to 10.7 millimeters. The value of a 2.5 carat diamond is determined by its quality, with higher quality diamonds (e.g., superior cut, clarity, and color grade) being more valuable than lower quality diamonds.