How is carat used to define purity of gold and weighing of other jewels?
Origin of Word Carat
The term 24-carat is neither related to carrot nor to clock but its relation is with a fruit pod tree generally known as carob, in greek the pod is named Ceratonia siliqua and the seeds of the pods are edible and useful,Both carat and siliqua from the scientific name Ceratonia siliqua were in use to define purity and weight of gold
Whats the relation between seeds and gold !
These seeds are light in weight about 0.2 gm.for each seed.In late Roman times a coin called 'Solidus' made from pure gold was introduced by Constantine I, which exactly weighted 24 seeds of these carob pod, the coin of pure gold approximately weighted 4.5 gm. Later other light weighted solidi (plural of solidus) were introduced for other purposes of trading and tributes which were 20, 21, 22, and 23 siliqua (weight of the carob seed).
So this is when carat was first used to denote weight of gold in a coin, later & now it defines the proportion of gold in a piece of metal. It means that any piece of jewelry when divided into 24 parts its karats are decided by how many parts of it are of gold. Suppose if jewellery is 100% pure without any impurities then it is 24k (24/24), 75% pure is described as 18k (18/24). Standard carat chart is in the image.
Difference in Karat and Carat
You must have noted that while describing purity of gold we have used a number followed by letter K, so this k represents karat and its origin is carat only. For convenience karat is used to describe gold purity and carat is used to express jewel’s weight.
Carats in Jewels
As mentioned earlier approximate weight of a carob seed is 0.2 gm or 1/5 gm. And this unit is used to express the weight of gem stones as a carat. Therefore 5 carat gemstone will weigh around 1 gm. A greater carat gemstone will be clearly larger in size but it should be noted that two different types of stones will not be same in size if there carat is same, because density of different gems are different. And further price of any jewel stone is mostly dependent on its carat apart from its cuts, flawlessness, and hue.