Carat or Karat
During a talk I recently gave on diamonds, we discussed the huge Koh-i-Noor diamond set in our Queen Elizabeth's crown (see above). When I gave its weight as almost 109 carats, a lady in the audience rightly asked "what does 109 carats" mean?
It was clear that this question also puzzled several other people in the audience who were confused about the way we use the same word to describe for example "18 carat gold" and a large "18 carat sapphire"
Our word "carat" originates in the arabic qirat and from the greek keration. It is believed to relate to use of the Carob bean as a weight. The regular, relatively uniform small size and weight of these beans made them very suitable to weighing very small and light objects like gem stones. Commerce between the Islamic countries in northern Africa and Iran and southern Europe in precious gems and metals was therefore carried out in carat units.
By the early 13th century, the word had been brought into everyday use in Spain and Italy, initially to describe the fineness of gold, but by 1344 it had also been adopted in Spain to weigh gemstones. Late as ever, we in England did not adopt the carat until 1477 when a law was passed not to put on sale any maner of base gold under the finenesse of xv111 carettes. But by the 1570s the term had been adopted in England for trade in diamonds and pearls.
Today, gold quality is universally measured against a base of 24 carat(ct) "pure", so that 22/24 parts= 22ct (92% gold), 18/24 parts=18ct(75% gold), 14/24 parts =14ct (58% gold) and 9/24 parts gold = 9ct (37.5% gold).
It is a pleasure here to be able to congratulate America for verbal clarity! There, the spelling karat is used for gold quality whilst carat is reserved specifically for gem weight, making it all quite clear and unambiguous (one carat is now defined as 0.2gm)
A perfect white diamond of more than 100ct weight is literally a "paragon". Mined in Brazil, the aptly-named white D-flawless Paragon diamond weighs over 137 carats and draws an interesting comparison with the supermodel Naomi Campbell with whom it became infamously linked around the Millenium.