According to at least one source, the terms karat and carat derive from Arabic and Greek words meaning "the fruit of the carob tree." Apparently, the seeds of the fruit, remarkable for their consistency, were used to balance the scales used by merchants at ancient bazaars.
Karats now refer to the ratio of gold found in a sample. There are 24 karats, so pure gold is 24 karats, and the legally permissible "karatage" for kan item to be called gold historically has differed from country to country. The US, for example, permitted 10 karat jewelry to be called gold even though it was well less than 50% gold.