There is often a lot of confusion about what 'carat' means when it comes to precious materials - and to confuse people further there's the alternative spelling of 'karat'! So I thought I'd give an explanation of what these terms mean and how they should be used.
Carats for gemstones and pearls
The term carat as it relates to gemstones and pearls is a unit of mass. A carat is the equivalent of 200mg, which can be divided into 100 units which are 2mg each. Since 1907 this definition of carat as a unit of mass has been adopted by most countries.
However, mass is not the only factor which determines the value of a gemstone. Gemstones (and especially diamonds) are generally valued using a combination of colour, cut, clarity and carats.
Carats for gold
As it relates to gold and its alloys, a carat is a unit of purity. It is measured as 24 times the purity by mass. 24 carat gold is effectively pure gold, while 18 carat is an alloy of 18 parts gold and 6 parts another metal, 14 carat is an alloy of 14 parts gold and 10 parts another metal ... and so on.
24 carat gold is very soft, but when gold is alloyed with another metal or combination of metals it becomes harder, so 18 carat is harder (and harder wearing) than 24 carat. This is why 24ct gold is rarely used to make jewellery which must survive ongoing wear and tear.
So what is a karat?
'Karat' is simply a different spelling of 'carat' as it relates to gold purity - the meaning is exactly the same.
In South East Asia, China, Europe and the US the spelling of 'karat' has been adopted to differentiate between the meanings for purity and mass. In Australia, Britain, Canada, Australia, New Zealand, South Africa and other countries which were historically part of the British Commonwealth, the spelling of 'carat' is used for both units of measurement (purity and mass) - the spelling of 'karat' is regarded as incorrect in these places.
In addition, in countries using the spelling of 'karat' the symbol used for gold purity is 'K' or 'kt', while in most standard English speaking countries the symbol is 'ct'. Again, the meaning is the same.
As a jewellery maker working in Australia I use 'carat' to refer to both gemstones and gold and the symbol ct for gold.
Do carats have anything to do with carrots?
No, nothing at all! The word 'carat' is believed to stem from the ancient Greek word for carob seeds, which were a unit of weight in classical times.