A carat is a unit of weight used to measure gemstones and pearls. One carat equals exactly 200 milligrams, which is about the weight of a small paperclip. When jewelers talk about a diamond ring, they often mention how many carats it weighs: a two-carat diamond weighs twice as much as a one-carat diamond.

Don't confuse carat with karat, which measures gold purity. A 14-karat gold ring refers to how much pure gold it contains, while a 2-carat sapphire refers to the stone's weight. If someone says they bought a “24-karat gold necklace with a 3-carat ruby,” they're describing both the purity of the gold and the weight of the ruby.

Bigger gemstones aren't always better or more valuable: a small, perfectly clear diamond might cost more than a larger, cloudy one. But carat gives us a standard way to describe and compare the weight of these precious stones.