The diameter of a one-carat round or brilliant cut diamond typically measures between 6.4 and 6.5 millimeters. The term carat refers to weight not size, so a one-carat diamond doesn't need to conform to specific measurements.
A diamond's quality and value are determined by several factors besides size and weight. These factors are known as the "4 Cs": carat weight, cut, color and clarity. The stone's cut refers to the shape of its facets and its appearance when viewed looking directly at its top. Color refers to the existence or absence of color. The highest quality diamonds are colorless like water. A diamond's clarity refers to the existence or absence of inclusions and blemishes, which are impurities that appear as white or dark spots.