A karat measures the purity of gold based on a scale of 1-24, with 24 karats being the purest. And actually, below 10 karats isn’t even considered gold. In jewelry, you will often find the karat weight stamped on a clasp or inside a ring. It is frequently abbreviated as 18K.
Gold is a very soft metal, so it is fortified with alloys (a harder metal, such as copper, zinc, or nickel) to make it stronger. 24-karat gold has the least percentage of alloys, whereas 12-karat gold is only 50% pure, for example.
The United States and Canada use the word karat with a “K” whereas in other countries they might use a “C” or simply use the millesimal fineness system instead.
A carat measures the mass of gemstones and pearls when weighing them, which is why you have a one-carat diamond (1ct) in your 18-karat (18k) gold ring.
The word carat is derived from carob beans which had such a consistent weight that gemstones were measured against it for a very long time. However, in 1907, the carat became the legal standard for weighing precious gems.
The global standard is: one carat = 200 milligrams. It is based on the metric system. For example, if your diamond weighs one gram, it would be five carats. Below one carat in weight, it will be referred to as a fraction, such as a “quarter of a carat.”
A carat can be divided into 100 points. So, for example, a half-carat stone (0.50ct) can be called a “fifty pointer.”