Diamonds are weighed in carats. It takes 5 carats to make one gram. The abbreviation for carat is ct. Carats are divided into 100 units, called points (as in decimal points). For example, One-fourth carat equals 25 points, and is written 0.25 ct.

Because nature produces very few large diamonds, the per-carat price of diamonds increases exponentially with size. For example, a one-carat diamond is worth much more than two half-carat diamonds, and very much more than four quarter-carat stones. These price leaps occur around important size divisions, called break points. For example, a 0.99 ct. diamond is priced on a different scale than one weighing a full 1.00-carat.

Diamonds are colour graded against a set of master diamonds that have been accepted and registered by the GIA or the AGS. Under standardized lighting conditions the diamond is judged by the amount of body colour present. Below is an example of colour grading chart:

Clarity is graded on a scale from Flawless to Imperfect. Diamonds below the grade of I₃ are not considered gems.