All you need to know about the carat measurement

Carats? Karats? Carrots? There are few things more confusing when touring the jewellery stores for a diamond wedding ring or necklace than trying to work out the terminology. And perhaps the most confusing piece of industry jargon is the use of carat.

The confusion comes from the fact that there are two separate measurements in the jewellery trade that look almost identical, but describe two totally different things.

Karat – with a K – is a measure of gold purity. Pure gold is 24-karat, but because gold is a naturally soft and malleable metal additions usually have to be mixed in to strengthen it, thereby reducing the purity of the gold. 18-karat gold is 18 parts gold to 6 parts additive (usually silver or copper). 12-karat gold is 12 parts gold to 12 parts additive. The sum will always be 24.

Carat – with a C – is a unit of weight used for gemstones. 1 carat is equal to 0.2g (200 milligrams), which means if you’re buying a diamond ring for your partner who weighs 70kg, you’ve got a 350,000 carat fiancé!

The reason that these two measurements have such a similar names is because they share a common origin. In the middle ages there was a German gold coin in common circulation called the Mark. It weighed 4.8g and was often split into 24 parts (0.2g apiece) to facilitate smaller transactions. The purity of the coin could be measured by the amount of gold segments remaining compared to those that had been replaced by inferior metals. A 24-karat gold Mark by purity had 24 carats’ worth of gold by weight.