Choosing a diamond

Diamonds
When we talk about diamonds, we need to minimally know the 4 Cs.
i.e. Caret, Clarity, Colour and Cut
Another factor to consider would be the HCA(Hollow Cut Advisor) estimate to measure light return.

1. Caret
The size/mass/weight of the diamond. Price per carat of diamond increases with caret size as larger diamonds are more rare, but more desirable to be used as gemstones.
More information can be found on: http://www.ajediam.com/diamond_carat_weight.html

The size in mm is an estimate of the round diamond top(plan view of a round diamond-see above) size, as a same caret diamond can be cut into different proportions. The cut will be explained later. Below is a rough visual estimate of the diamond sizes.

2. Clarity 
It is a measure of the defects of a diamond, which can be due to it's structural imperfections(perhaps during it's formation?) or crystals of a foreign compound/diamond. The higher better clarity, the more expensive it is.
click on image to enlarge

Inclusions : Clouds, Feathers, Included crystals or minerals, Knots, Cavities, Cleavage, Bearding, Internal graining

Blemishes: Polish lines, Grain boundaries, Naturals, Scratches, Nicks, Pits, Chips, Breaks


3. Colour
The measure of chemical imperfections of a diamond. The more colourless a diamond is, the more pure it is and likely to be more expensive. In some cases, a rare-coloured diamond is valued higher than a normal one. Examples are certain pink diamonds. As I'm not very experience with coloured diamonds, I will only be discussing on white diamonds. Below is the grade scale of white diamonds. And a D-colour diamond is being seen as the purest and most expensive of them all.

Grade scale:

Real diamond look- click image to enlarge:

Adapted from: http://www.a1-diamond.com/Diamond-color.html


4. Cut
Diamonds can be cut into different patterns, and need to be cut into a certain symmetry to allow as much light return as possible. The cut grade defines how "symmetrical" and "correct proportion" that a certain diamond is being cut into, and it's grading terms differ between GIA, AGS and other grading associations.

Cut proportions
Below is an example of how a differently cut round diamonds will affect light return.

Cut patterns

Read more @ http://www.a1-diamond.com

Apart from this, there are other considerations that need to be taken into account, such as the HCA value.

HCA - adapted from Pricescope
The Holloway Cut Advisor (U.S. Patent 7,251,619)  estimates a round diamond appeal based on its potential Light Return, Fire, Scintillation and Spread.

All that you need to do basically is to key in Depth %. Talbe %, Crown % or Angle, Pavillion % or Angle, Cutlet % in http://www.pricescope.com/tools/hca

Below image is from there too.

Most people prefer stones that rate 1-2 on a scale where: 

->0-2 Excellent

->2-4 Very Good

->4-6 Good

-> 6-8 Fair

-> 8-10 Poor

Note: 0 is almost impossible since many of the factors conflict.

It's a lot of knowledge in this area, and you can find alot of information from the above stated site.