The weight of a gemstone is measured by carats. The definition of a carat has changed over time, but the international standard for the weight of 1 carat is 200 milligrams, according to Zales Diamond Store. The diameter of a 1-carat diamond is about 6.5 millimeters.
Jewelers often refer to carats in one-quarter increments. Diamonds vary in size and can be measured as several carats or as low as a quarter of a carat, Zales states. When in reference to jewelry that weighs more than one carat, the carats may be measured in total carat weight, abbreviated as TW. Total carat weight is the total weight of all of the stones in a piece of jewelry combined.
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.