Why is gold jewellery always described in karats?

That’s karats, not carrots. (But they sound the same!) Gold is soft, so it is often alloyed (mixed) with other metals such as silver, copper, or zinc. The amount of gold in an item is measured by karats. Pure gold is 24 karats. If something is 12 karat gold, 12 parts out of 24 (or half) are pure gold.

Originally, a karat was based on the weight of a seed from the carob plant. (Carob is sometimes used as a substitute for chocolate.) The Ancient Greeks used carob seeds as a unit of weight.