No two diamonds are alike. So how do you choose? During creation, diamonds are subjected to natural processes which affect their clarity, carat weight and colour. Then, a professional craftsman will cut the stones to release their brilliance. The 4Cs determine the value of each diamond. Understanding them and deciding which you value most will enable you to make an informed choice.
1. Clarity
A diamond is not made entirely from carbon. Usually, the formation process allows trace minerals, fractures and other imperfections to appear in the stone. Clarity refers to how many or how few of such inclusions occur within the diamond. There are different grades of clarity represented on a scale that ranges from flawless (FL) to inclusion (I).

FL
Flawless
  IF
Internally flawless with minor surface blemishes
  VVS1 & VVS2
Very, very small
inclusions (not visible
to the naked eye)
  VS1 & VS2
Very small inclusions
(not visible to the
naked eye)
  SI1 & SI2
Small inclusions
(not visible to the
naked eye)
  I1, I2 & I3
Inclusions (visible to
the naked eye)
Almost all natural diamonds contain inclusions. Seen under 10-times magnification, inclusions may resemble tiny crystalline, cloudy or feathery birthmarks. They do not necessarily mar a diamond's beauty nor do they reduce its inherent durability. However, the number, colour, type, size and position of these birthmarks affect the value of a diamond. After all, such inclusions interfere with the passage of light through the stone, resulting in a less sparkling gem.
2. Carat weight
The weight of a diamond is measured in carats. One carat is equivalent to 100 points or 0.2 grams. A 0.75 carat diamond is the same as a 75-point stone or a ¾ carat diamond.
 
Large diamonds are found infrequently in nature and therefore are most highly prized. Also, big diamonds exhibit colour and brilliance better than smaller stones. This marks up their value when well-cut. One 2 carat diamond of a certain quality is always more expensive than two 1 carat diamonds of the same quality.
3.

Colour
Diamonds can be found in a range of natural colours from colourless to light yellow. Most diamonds contain traces of yellow or brown and are still desirable, while colourless diamonds are rarest. But this is not to say that colourless diamonds are always the most expensive. They are however the most valuable because the less colour a diamond has, the more white light can pass through and be dispersed into the colour spectrum. This gives a more brilliant diamond.

Diamond colour is rated according to the GIA letter scale, starting at D and continuing through the alphabet.

D to F No detectable colour tone to naked eye
G to I Nearly colourless to the naked eye, but a trained
eye may detect a slight yellow shade
J to M Detectable trace of colour to the untrained eye,
but still desirable for jewellery
N & below Noticeable yellow tint
4.

Cut
Many people confuse a diamond's cut and its shape. Cut refers to the arrangement of the stone's facets. Regardless of its shape, a well-cut diamond commands a higher value as it reflects light better and produces a more fiery sparkle. On the other hand, a poorly-cut diamond will appear lifeless regardless of its colour and clarity.

  Too shallow
Light escapes through the bottom
  Too deep
Light escapes through the sides
  Perfectly cut
Reflects more light on each facet for more brilliance on top

The refractive abilities of a diamond demand precision geometric calculations to unleash its fire. Fortunately, you do not have to be a trained jeweller to appreciate a good cut. When choosing your diamond, hold it up and see how light dances around it.