The 4 C’s of the Diamond Quality are: carat(weight), cut, clarity and the color.
Depending on thes 4 C’s properties and the different combinations of all determines how much a diamond is worth.

1. Carat Weight

There are 100 points to a carat, so that a diamond of 75 points weighs .75 carats.
Carat is often confused with size even though it is actually a measure of weight.
One carat on a diamond is equivalent to 200 milligrams.
Wrong is that a 1-carat diamond costs exactly twice the price of a half-carat diamond.
So two diamonds of equal weight can have very unequal value, depending on their cut, color and clarity.
The most popular sizes of diamonds are 1/2 carat, 1-carat, and 2-carat-

2. The Cut

Refers to the angles and proportions of a diamond.
A Cut of a diamond actually refers to tow separates aspects of apperance: The first ist the shape and the secend is quality. Depending of the workmanship of manufacturing.
You will find a differend kind of shapes , but the most popular are round brilliant, marquise, oval, princess, pear, heart and emerald.
Go for the Ideal Cut ff you want perfection regardless of cost
Go for very good to excellent make ff you want quality and beauty
Your choice is a matter of personal preference. When cut to good proportions, the diamond is better able to reflect light, creating more scintillation, more sparkle.
3. The Clarity
Flawless: perfect inside and out. Flawless or Internally Flawless (very rare and expensive)
Internally Flawless: minor surface blemishes;
VVS1, VVS2: have very very small inclusions. VVS1 inclusions can only be seen through the pavilion. VVS2 inclusions are more visible. (still flawless to an untrained eye with a 10x loupe)
VS1, VS2 = very small inclusions. (certainly flawless to the naked eye, even to a diamond grader) VS1 inclusions are harder to see than VS2
SI1, SI2, SI3 = small inclusions (some SI3 stones will also be flawless to the naked eye)
I1, I2, I3 = Imperfect - inclusions visible to the eye. I don't recommend them for engagement rings because they lack brilliance.

The greater a diamond's clarity, the more brilliant, valuable and rare it is.
4. The Color
Refers to the degree to which a diamond is colorless.
Diamonds are found with a range of colors but generally the more colorless a diamond appears, the greater ist aesthetic appeal.
The color is the most importand feature of the diamond's appearance after its shape.
A color scale established by the Gemological Institute of America (GIA) ranges from D (colorless) to Z.
K-Z are waremer colored Diamonds
D–J look white gold or platinum
E - F still colorless to any eye