The weight of a diamond is expressed in carats. The word carat originated from the carob tree or Ceratonia siliqua. The tiny seeds of this tree are well known for their uniformity and consistent weight. Traditionally diamonds and gemstones were weighed against these seeds until the system was standardized, and one carat was fixed at 0.2 grams. One carat is divided into 100 points. A diamond weighing one quarter of a carat can also be described as weighing 25 points or 0.25 carats. Points are generally not used to describe weights over one carat.

It is very important not to mistake carat weight as referring to the dimensions of a diamond. It refers to weight only.

Size of a diamond Vs Rarity:
The rarity of a diamond is greatly affected by its size. The rarity of a 1.00 carat diamond is much greater than twice that of a .50 carat. Although it only weighs twice as much, the 1.00 carat is statistically much more difficult (rare) to mine than the .50 carat.