A diamond's weight is measured in units called carats, each equivalent to approximately one fifth of a gram. The word derives from the name for the tiny seeds of the carob tree, which were traditionally used to weigh diamonds in the ancient Middle East. A carat is often expressed in points (1/100ths of a carat) so that a half-carat diamond measures 50 points. The carat weight of a diamond plays a major part in determining its surface dimensions. As you would expect, larger diamonds are rare and are therefore far more expensive than the equivalent weight in smaller diamonds.