Carats and Karats
The term carat is used as both a unit of mass -- especially for gemstones -- and as a measure of purity for gold. In the United States and Canada these terms are spelled differently -- as carat for gemstones and as karat for Gold -- to avoid confusion.
>Interestingly, the term karat also derives from the name of the carob seed. So how did they get from the weight of the carob seed to the purity of gold? And why is pure gold defined as "24 karat"? A precise answer is not easy to find, but the connection appears to derive from the Roman Emperor Constantine, who introduced a gold coin that weighed exactly 24 carob seeds.
Under the karat system of gold purity, pure gold is 24 karat or 24k, 18k is 75% pure and 12k gold is 50% pure. This system is gradually giving way to the Millesimal fineness system which expresses purity in terms of parts per thousands of the pure metal by mass. For example, an alloy containing 75% gold is denoted as "750". Thus 18k gold would be marked as "750" and 22k as "916" under this system, or 91.6% gold. It is certainly a simpler and less confusing method.
In Thailand, as in most Asian countries, gold is very important as a medium of exchange and a store of wealth. The Thais still use a traditional measurement of weight for gold, the "baht." This term can be a bit confusing for foreigners, since the Thai currency is also called the baht. One baht of gold is equal to 15.2 grams, slightly less than one half a troy ounce (16.2 grams).
Fortunately for the gemstone industry, nearly the entire world has standardized on the carat as the measurement of weight for gemstones. We do occasionally get requests for gemstones measured in rattis, an Indian measurement where 1 standard ratti is 182 mg. But the main confusion any gem dealer encounters with carats is the inexperienced buyer who thinks of the carat as a size measurement.
Different varieties of gemstones differ considerably in their density or specific gravity, and different cuts also affect how large a gemstone appears when viewed face up. Always check the dimensions in millimeters when you're buying a gemstone and don't rely on the carat weight alone.
This is very important some people will look at diamonds and mentally convert the carat size to sapphires or rubies where as the stones are the same weight ( carats ) but are very different is size (mm), its always important to look at each stone taking into account its depth as well as the length and width.