As discussed in the previous post, the term ‘carat’ is used to measure both the weight of gemstones and the fineness of gold, based on the weight of the seed of the carob tree. In this post, I am only examining the term as it applies to gold.
The fineness or purity of gold is measured in carats. Pure gold is 24 carats (24 ct), but is very soft and so others metals are added. The highest quality gold used for jewellery is 22 ct. The number of the carat tells us what percentage of other metals has been added to the gold. The higher the number, the less alloy has been added. For example, if we take 9ct gold, this means that 37.5% (9/24) of the metal is gold, the rest alloys, while 18ct gold (18/24) means that 75% of the content is gold.
The idea of a carat being made up of 24 parts is thought to come from Roman currency introduced around 309AD. The gold solidus was comprised of 24 siliquae. The solidus became the standard for pure gold as its weight was constant. Twenty four carat seeds (silique in Latin) weighed the same as a solidus.
The weight standard of Frankish and Anglo-Saxon gold coins changed at the end of the 6th century, with the carat replaced by locally grown barley grain. In the 13th and 14th century, England replaced barley with wheat grain. Other countries made similar changes but the concept of a gold standard comprised of 24 seeds did not change in many countries.
Commonwealth countries that still use or used to use the carat system continue to use the term ‘carat’ when referring to the purity of gold. It is abbreviated to ‘ct’. The US, which also uses the carat system, adopted the term ‘karat’ (k) to differentiate it from the term used to weigh precious stones. Canada accepts both carat (c or ct) and karat (k or kt).