Colour: You can generally see the flashes of colour in a diamond when it is exposed to sunlight. A diamond’s colour is judged according to a grading system which determines its value. This is how the grading system works.
D, E, F = colourless
G, H, I, J = near colourless
K, L, M = faint yellow
N, O, P, Q, R = very light yellow
S, T, U, V, W, X, Y, Z = light yellow
Grade D is the most expensive, and as you work your way down to grade Z the prices drop.

Cut: The accuracy of how well the diamond is cut will determine the brightness of the colour and how well it sparkles, so it is vital that the diamond is cut accurately. Still, the cost is mostly determined by the portion and the finish. Diamonds can be cut into a variety of fancy shapes; common ones include pear, marquise, emerald, and oval shapes.

Clarity: Clarity refers to the amount of flaws a stone has. An inclusion is an internal flaw, while a blemish is an external flaw. Of course, you will be hard presses to find a flawless stone, so this is how clarity is judged:
FI = flawless
IF = internally flawless
WS1/WS2 = very very slightly imperfect
VS1/VS2 = very slight imperfect
SI1/SI2 = slightly imperfect
I1/I2/I3 = imperfect
If a stone is rated as I1/I2/I3 that means the imperfections are visible to the naked eye. If possible, avoid buying these diamonds.

Carat: When people ask what size a diamond is, they are generally referring to how many carats it has. A carat defines the weight of the stone. Slight changes in size can affect the price tremendously.

Sometimes a fifth C is added, referring to a diamond certificate. A diamond certificate provides you with the 4Cs – colour, cut, clarity, and carat – and, therefore, the diamond’s value. Get one – it’s a small price to pay for peace of mind.