A measurement of gemstone weight, most often associated with diamonds. Carat is often thought of as the size of a stone, which when comparing gemstones of the same type is accurate. However different gemstones have different densities, which means two gemstones stones with the exact same measurements might not have the same carat weight.
For example, a diamond that measures 8 x 8 x 4mm weighs 1.62 carats. An amethyst of the exact same size weighs only 1.22 carats. As long as you are comparing “apples to apples,” such as when you are shopping for diamonds, it’s ok to think of carat as a measure of stone size.