The modern classification of carats with diamonds came in 1913, and any antique jewelry prior to that date have likely have a slightly larger carat. It is important to remember this if you are dealling with antique jewelry, you may get more than you pay for, per carat. Prior to 1913, an American carat was different from the French, or the Indian caract, which caused confusion in the market. Today the term carat refers to a metric scale in weight. Since 1913, a carat weighs 200 milligrams, or one fifth gram. Jewelers usually describe a carat in terms of points: that is, each gram earns one point so a 100 gram diamond is one half a carat.
A carat is measured in terms of weight, not size. It may be that a heavier diamond is smaller than a lighter stone with less weight, such as an emerald. Comparing points between a diamond and emerald, also points out a difference as an emerald weighs less than a same size diamond. This is true of all minerals as it would be otherwise in nature: that is, the same sized diamond would be dwarfted by a same size piece of wood. In this way, the ruby is heavier, and the emerald is lighter than the same sized diamond. Weight is the determining factor.
While jewelers refer to diamonds in terms of points, the prices and description of them to the public are measured in carats. Jewelers often refer to diamonds in terms of one half an ounce a caract up to five ounces at the top end. The various cuts for diamonds are emerald, Marquis, Pear, and Brilliant. That is if the stone is cut properly it will be cut along the sizes of these forms for easy comparison to other diamonds. Prices of these stones are usually measured in per carat sizes, and are sold for the highest price per car. Stones that are progressively of lower value will sell for that much less.
As example a rare quality diamond of one ounce may sell for $20,000 while a stone of the same weight but inferior quality or cut may sell for half that price. It should be noted that price increases more quickly with diamonds than other jewels as size and weight. Another means of illustrating this is to compare a one half carat diamond of particularly good quality that might sell for 5,000 while the per caract costs for a one half carat of lesser brilliance or overall quality may sell for $3,000. Diameters and corresponding weights of round brilliant cut diamonds can range from a 2.58 mm diamond that is one sixteenth ct, up to a fourteen mm stone weighing ten carats.
You will find that diamonds of the same size and weight often vary in their value, that there are disproportionate leaps and hesitations along the way up depending on the particular brilliance of the stone.A top quality five carat diamond would be worth up to ten times, rather than five times that of a one carat stone of lesser cut and quality, brilliance. And the manner in which a stone is cut may well affect the price of the finished diamond. In addition that is the value in the term spread, which is the flat portion on the top of the stone. If cut too shallow, the spread will be wide but the diamond will be out of proportion and be discounted. The middle portion of the stone below the spread is the girdle, which is adversly affected if the spread is cut too low and the girdle is made too thick.
Derek Dashwood finds the study of diamonds and gemstones fascinating and how one small stone can take your breath away at diamonds and gemstones