First things first, if someone offers you a 10 karat ring or a 10 carat diamond, take the diamond. The ring might be worth $100, the diamond $100,000. (disclaimer: this kind of offer rarely happens)
What is a carat?
A carat is a unit of weight principally used for measuring diamonds and gemstomes. Each carat is exactly equal to 200 milgrams, or 0.2 grams. The origin of the word comes from the carab seed, which is a seed that was thought to have very little variance in weight and could therefor be used as a consistent form of measurement. In reality however, there was a significant variation in weight between the seeds and as a result of this dishonest classical gold traders could have two sets of carab seeds – a heavier one used for buying (heavy seeds make the same piece weigh less carabs) than for selling. Clever customers would pick up a handful of their own carab seeds on the way to sell their gold to ensure fair treatment.
What is a karat?
A karat (usually spelt with a K to avoid confusion with carat) is a measure of the purity of gold. A karat of gold refers to the number of parts per 24 that are pure gold. As an example, 24 karat gold is 100% pure and 10 karat gold jewellery is, by weight, 10 parts gold and 14 parts other metals or 41.6% pure.
The origin of karat as a measure of purity actually originated from the use of carat as a measure of weight. There is some debate over whether the use of karat came from the Roman coin the “solidus” or the German coin the “mark”, but whichever coin it was the story is the same. The mark/solidus weighed 24 carats or 4.8 grams so people would refer to the coin as 24 carat. When a coin of the same weight but less purity was minted people referred to the new coin as a lower carat even though the weight remained the same. A 12 carat coin for example would weigh 24 carats, but only contain 12 carats worth of gold (as it was only 12/24 = 50% pure). From there it became understood and accepted that carat would be used as a measure of purity with a base of 24.
The conventionally spelling when using carat to refer to a purity of gold was changed to karat to avoid confusion, although the spelling of carat is still often used to purity.
What is a carrot?
A carrot is an orange vegetable worth nothing near gold or diamonds unless you are sufficiently hungry.