Cut
is the most important for the brilliance of the stone. A poorly cut larger diamond can be worth considerably less than a smaller, properly cut diamond.
Colour
The closer to the colour of water, the better, because when light enters a colourless stone, it can easily emit its brilliance. Of all the gemstones, diamonds are the only ones where the colourlessness render them more valuable. Diamonds carrying a D, E, or F rating are the most expensive, and the colour scale goes down the alphabet to Z. Changes in colour are minute, but the price is not. The average stone carries an 'I' or 'J' grading for its colour.
Carat
The weight and therefore the size of a diamond is expressed in carats. One carat is divided into 100 points. There are 142 carats to an ounce. Remember, bigger is not necessarily better.
Clarity
Most diamonds contain minute traces of naturally occurring non-crystallized carbon called inclusions found on the outside of the stone, or trapped inside the crystal of the diamond. Most of these can only be seen with 10X magnification. Inclusions interfere with the dispersement of light and therefore the diamond's brilliance. The fewer, the better, and the rarer the stone.