The origin of the word carat is in the seeds of the carob tree, which were used for the weighing of precious stones in ancient times due to their weight uniformity. A diamond carat is a measurement of its weight not its size, and is not to be confused with Karat, used for determining the purity of gold. Large (high carat weight) diamonds are found less frequently than smaller diamonds and their price is generally--but not always--proportionate to that size.
Colored diamonds tend to appear naturally in smaller sizes compared to other diamonds and gemstones. In fact, very few pink diamonds from the Argyle mine in Australia are over one carat in size. At last year's Argyle tender, the largest pink diamond was 2.03 carats and most of the stones were between a third of a carat to one carat in size. Because colored diamonds have a higher price tag and are more readily available in smaller sizes, there is an active sub-carat collector market for these stones.