The age-old question: "Carat or Karat, why the difference in spelling?"
Few people actually know that both these words trace their ancestry to the carob bean. Carob trees have been around since ancient times, and are often used as a chocolate substitute. They produce a bean which is incredibly consistent in weight, hence its' quick adoption for use in measuring precious gems, diamonds, and gold.
Although both words share the same origin and pronunciation, they now have different meanings. While carat is used to weigh gemstones and diamonds, karat is used to denote the purity of gold. The international standard (since 1913) for one carat is 200 milligrams (also known as a "metric" carat). Carat is often abbreviated to c. or ct.
Twenty-four karat (24kt) is one hundred percent (100%) gold. Since gold is a very soft metal, it is combined with other metals, also known as alloys, for more lasting jewellery. For instance, 22karat gold is 22 parts pure gold and 2 parts other metals. Since the 22 parts is out of a possible 24, it is also commonly marked as "916," or, 91.6% gold content (18kt or 0.750, 14kt or 0.585, 10kt or 0.417, 9kt or 0.375). Karat is abbreviated to k. or kt.
The difference between the different colours of gold in the same karat is the alloy used. For example, the primary alloy metal used for white gold is nickel while the primary metal used for yellow gold is copper. Various shades of colour are created by the other alloys used in the composition of each karat.
One carat for a gem stone is equal to "100 points." Thus you might have a gem stone under a carat designated as 0.21 carats -- which equals 21/100. Or you may have a stone that is over a carat which could be designated as 1.51ct or 151/100. All gem stones are referred to in percentage of 100 points. Thus 1ct would be 100 points. Gem stones are seldom exactly 1 carat for which the fractions are very important. This is also why smaller stones in the same quality are always worth less than stones in the same quality which are larger. The larger the stone, the larger the value per carat.