In ancient times, people used carob seeds to balance the scales which are the same in weight and equally-shaped. The word "carat" is originated from the carob seeds. "carat" is different from "karat" which is used to specify the purity of gold.
A carat is the universal unit to measure the size of a gemstone. It is very easy to determine compared to the other 4C's. The carat-weight has nothing to do with the appearance or quality of a diamond. It just indicates the size of a diamond.
There are five carats in a gram. One carat is subdivided into 100 "points". If we take an example, a diamond measuring 75 points is actually 0.75ct or Â¾ carat. The symbol of carat is written as a CT or ct. a total carat weight (TCW) is measured when jewellery is set with diamonds.
When rough diamond are cut and polished, it loses as much as 2/3 of the total carat-weight. There is a possibility that two diamonds of same carat-weight may differ in value due to its cut, color and clarity. As larger rough gems of high quality is rare to find than smaller rough gems of high quality, a single two carat diamond may have more value and price than 2 one carat diamonds of the same quality.
It is essential to know that not only carat-weight determines the size of a diamond. There are two other factors also reflect the size.
When a diamond is set into a ring or pendant, it is viewed from the top. Therefore, when a diamond is cut deeply, the proportion of weight "hidden" in the depth may greater than a well-cut diamond which in turn, results in a smaller diameter.
In a well-cut diamond, maximum light is reflected from the top and it results in appearing a diamond much larger than actual.
How to choose right carat weight?
As carat-weight has nothing to do with the beauty and quality of the diamond, this question has no direct answer. A size difference can be noticed by everyone. Also, you must keep in mind that a weight of a diamond directly proportional to cost of a diamond.
In most of the cases, it depends on a person's individual choices of buying diamonds of different carat diamonds. If you want to buy 1 carat-weight diamond, a high quality half carat diamond also may disappoint you.
It will be preferable to consider other aspects of a diamond like cut, shape, clarity and colour than carat-weight as these factors determine the quality and beauty of a diamond.