International measure of weight for most gemstones including cut and polished Opals (1gm = approximately 5 cts)

Metric system of measurement for gemstones where one carat equals 200 milligrams, or 0.2 grams. Each carat is divided into 100 points, so a 75 point stone is ¾ of 1 carat, a 50 point is ½ carat, etc. Note that gemstones can only be weighed for certification when un-mounted.