Difference between carat (ct) and karat (kt)
The same word, even though with a different spelling, is used to describe the weight of a diamond (carat, abbreviation ct) and the purity grading of gold alloys (karat, abbreviation kt).
The carat (ct)
The weight of gemstones (including diamonds) is expressed in carats (1 carat = 0.2 grams). One carat is divided into 100 points. In practice, the weight of a diamond is indicated to two decimal points, e.g. 1.35 ct (which represents an accuracy to 0.002 grams). The weight of a 1.35 ct diamond can be, therefore, expressed as "one carat thirty-five". A weight of 0.35 ct can be expressed as "thirty-five points".
The word "carat" comes from the Greek "kerátion" or from the Arabic "kharrūb", which are both names for the seed pods of the Carob tree. In the past, the gemstone traders noticed that the seeds of the Carob tree have an almost constant weight of 0.197 grams. They adopted this weight as the standard unit of a gemstone weight. In the year 1907, the metric carat, representing one fifth of a gram, was recognized as a standardized unit of weight of a gemstone.
The value of a diamond cannot be simply calculated as a multiple of the carat price and the carat weight. A 1.20 ct stone is much more rare than a 0.30 ct stone. Therefore, the price per carat of a 1.20 ct stone will be much higher than the price per carat of a 0.30 ct stone. In fact, there are several carat weight boundaries after which there is a sudden leap in the carat price of a diamond.
The karat (kt)
The word karat represents the ratio of pure gold / other metals in a gold alloy. It is expressed in karats (or thousands). Pure gold is 24 karat or 1000/1000, 18 karat gold is divided 750/1000 or 3/4 pure gold and 1/4 other metals.