Carat refers to the unique unit of weight measurement used exclusively to weigh gems and diamonds. One carat is defined as 200 milligrams.
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. Example; a single 2-carat diamond will cost much more than two 1-carat diamonds. Very large diamonds with good colour and clarity are very rare.
The modern carat system started with the carob seed. Early gem traders used the small, uniform seeds as counterweights in their balance scales. The carat is the same gram weight in every corner of the world.