You have probably read about karats and carats in reference to both diamonds and gold, and for one it's a unit of weight and the other its a unit of purity! Confusing right? We can set you right.
Firstly, in Australia, the UK and many other places, we use the "C" carat to refer to both diamonds and gold. The "K" karat is the American spelling. We shorten carat to ct for both as well.
When looking at diamonds and other gemstones, a carat is the unit of weight that they are measured in. 1 carat is 200 milligrams (which is 0.200 grams). When talking of diamonds, the higher the carat, the larger the stone. This generally correlates to the higher the price as well, but there are a few other things that go into this, such as purity and colour.
When talking about gold, a carat is a unit of measurement that tells you how pure the gold is. 24ct is pure gold, but often pure gold is too soft to create some types of jewellery, so another metal, like silver or copper is mixed. 18ct gold has 18 parts gold to 6 parts of another metal. 14ct gold has 14 parts gold to 10 parts of another metal. And so on.
If you're wondering why 24 became the magic number for gold purity, it actually has German roots. About a thousand years ago the gold coin, called a mark, weighed 24 carats, and the purity of this gold was expressed in the number of carats. And we still use that measurement today.
We like to use 9ct gold in a lot of our jewellery, because it's a nice strong metal that is reasonably priced, so it makes a great choice for most styles. But we also do like to use 18ct gold for all heirloom jewellery, like engagement rings and wedding bands, as 18ct gold can be endlessly remodelled and created, so it has a long life ahead of it.