A diamond cut to proper proportion allows the greatest amount of light to be reflected, for maximum brilliance.  

Most diamonds appear colorless, but there are many subtle differences in shade. The closer a diamond is to having no color the more valuable it is. 

( unless it is a rare "fancy" color )

The clarity of a diamond is determined by the amount and location of imperfections in the diamond. The fewer imperfections in a diamond the better: A flawless diamond is extremely rare and valuable.

A carat is a measure of weight, which also indicates size. Carat weight is often referred to in points. 

1 carat = 1.00 points

1/2 carat = .50 points 

       and so on...