The value of a diamond can vary widely from one to another. Experts evaluate diamonds for their rarity and beauty using four primary guidelines, known as the 4 C's - Cut, Color, Clarity, and Carat Weight.
Cut: refers to the proportions and its effect on the overall appearance of the diamond. A diamond cut with excellent proportions will reflect the most light back to your eye. A diamond that is cut too deep or too shallow reflects more light in other directions.
Color: refers to the hue of the diamond. The color grading scale for white diamonds ranges from D-colorless to Z-light yellow or brown. The most valuable white diamonds are those with the least color. Fancy colored diamonds such as yellow, pink, and blue are graded on a different scale.
Clarity: refers to internal inclusions and surface characteristics within a diamond. The amount and location of these flaws reduces the brilliance and value of the diamond. The clarity grading scale ranges from Flawless (F) to Included (I).
Carat: refers to the standard unit of weight for gemstones. 1 carat is written "1.00 Ct" and can be called "100 points", so a diamond of 25 points is 0.25 Carats or a "quarter carat". The larger the diamond the more rare it is. Because of this, cost increases with carat weight.