A 0.5 carat diamond is a diamond that weighs approximately 0.5 carats, which is equal to about 100 milligrams or 1/20th of a gram. The size of a 0.5 carat diamond will depend on a number of factors, including the diamond's shape and cut, as well as its clarity and color. In general, a 0.5 carat diamond will be small to medium in size, with a diameter that ranges from about 4.3 to 5.3 millimeters. The value of a 0.5 carat diamond will depend on its quality, with diamonds that are higher in quality (e.g., with a better cut, higher clarity, and higher color grade) being more valuable than diamonds that are lower in quality.