More than one diamond, the carats may be described in terms of a total carat weight (TCW). This is the combined total weight of all the stones in the piece. Diamonds can range in size from a fraction of a carat to several carats. Given the rarity of large stones, however, the price increases rapidly with size; therefore, a single 2-carat diamond will cost more. Carat is the unit or symbol used to describe the weight of any gemstone, including diamonds. The definition of a carat has changed over time since 1913 the international standard has been 200 milligrams or 1/5 of a gram. Often, jewellers describe carats in 1/4 increments. In jewelry pieces with much more than two 1 carat diamonds. Very large diamonds with good color and clarity are very rare. Expect to pay a premium for stones that are above a full carat weight. For example, a .95 carat diamond will cost a bit more than a .90 carat stone, but a 1 carat stone will cost significantly more than a .95 carat stone.
According to Wikipedia, the term diamond carat comes from the Greek word meaning “fruit of the carob tree”
In ancient times, the seeds of the carob pod were used as standards for weight measurements, since the size of their seeds were almost always identical. This provided a natural standard of measurement.