Carat Explanation

Carat refers to the weight of a diamond.
 

As with all precious stones, the mass - and therefore the size - of a diamond is expressed in carats. The carat has an interesting story. Centuries ago, the gem dealers of the Middle East used the seeds of the carob tree (carob became carat) to balance their scales. These seeds were called "keration" (little horn) in Greek, because the pods in which they grew were shaped like horns. These seeds were surprisingly uniform in mass; on ancient scales the balanced exactly, and even the best modern scales cannot detect more than one three-thousandth of an ounce difference between the seeds. Diamond were traditionally weighed against these seeds until the system was standardised and one carat was fixed at 0.2 grams (one fifth of a gram). A carat is divided into 100 "points" so that a diamond of 75 points is described as a three-quarter of a carat or 0.75 carat (ct.).
 

Since larger diamonds are found less frequently in nature, a single 1-carat diamond will cost significant more than two 1/2-carat diamonds, assuming the colour, clarity and cut are the same.