Many assume “carat” to be a unit that measures diamond size. Instead, “carat” measures diamond weight. One “carat” equals 200 milligrams, or 20%of a gram.
Not all diamonds of the same carat-weight appear to be the same size, however. Apparent size depends upon the shape of the rough stone and precisely how it was cut and finished. For example, a one-carat diamond that is unusually deep may appear smaller (looking down from the top) than a better-proportioned, more expertly cut one- carat stone (see “Cut”, below).
Gemstone weight also is expressed in “points:” One carat equals 100 points. So a half- carat gem is “50 points,” a quarter-carat gem can be described as “25 points.” A gem greater than one carat can be described in both carats and points, for example as “1.63 carats,” or “one carat, sixty-three points.”
Diamond price is founded on rarity. Obviously, large diamonds are radically more rare than small ones. Therefore, the price of a diamond that weighs, say, a full carat is dramatically greater than twice the cost of a similarly graded half- carat diamond.
The combined carat-weight of all the stones in multi-gem jewelry frequently is described in terms of the total carat weight (TCW) of all combined stones in the piece.
(Don’t confuse “Carat,” [gemstone weight] with “Karat,” [percent of pure gold in an alloy]. For example, 24-karat is 100% gold; 12 karat is 50% gold; other metals comprise the remaining 50% of the alloy.)