Carat is the weight unit equal to a fifth of a grams. 1 carat equals 0.2 grams.
The origin of the word is from the Greek word KARTION which means carob seed. In ancient times a diamond's weight was measured against the weight of the carob seed.
In commerce, when processing a raw diamond, we tend to keep the maximum weight while emphasizing the critical weights such as 0.50, 0.70, 1.00, 1.50 ...as well as the level of clarity and the work of the diamond.
A price of a diamond is a price per Carat which means if a stone cost 2000$ and it weighs 1.5 carats. It price will be 3000$.
In trading, a portion of a carat is called "points". i.e a stone weighing 0.10 carats is equal to 10 points.
A diamonds Proportions refers to the attribute of the height and the width of a diamond. Perfect proportions are when they fit "the ideal of Tolkovsky polishing" and they are in fact responsible for the three types of diamond light reflections:
Brilliancy – a white light which is reflected from within the stone or its surface. Fire – a white light breaking to its nature colors: blue, yellow, red, etc ... Scintillation – a white light that is reflected as a result of movement or displacement of the stone.
Typically, we rank the color of a diamond by letters, D being the highest color and Z the lowest. The price of a diamond will be affected by its color.
Colors below the Z, will be called FANCY colors.
Fancy colors will be more expensive then colors ranked D-Z. this is due to the rareness of Fancy colors and them being more desirable.