It can be pure, and as one customer told me, it can also be “diluted”. Acccording to the French and the Eastern Europeans anything less than 18 karat is “polluted” and cannot be considered as Gold at all.
Gold comes in various standards 24, 22, 20, 18, 15, 14, 10, 9, and in Germany even 8, though again, to the French, this is tinsel.
In Australia, the most common standards are 18, 14, and 9. The numerical standard for 18 karat is 750 and this literally represents its purity: it is 75.0% pure, By the same token, 375 is the numerical standard for 9 karat and it is therefore 37.5% pure. In the case of 9 karat Gold the remaining 62.5% is what gives 9k its hardness. Pure Gold is notoriously soft and so the attraction of the smaller standards is that they are more durable, especially for things like chains and rings.