Diamond Color

Colorless and near-colorless diamonds are the most valuable. Though most diamonds may appear colorless to the naked eye, the majority of diamonds contain slight traces of yellow or light brown when viewed under a jeweler's loupe. Depending on the stone's size, a single increase in color grade can boost the value of a diamond by thousands of dollars per carat. A traditional engagement diamond is usually colorless or near-colorless. 
In nature, diamonds can also occur in shades of red, pink, blue, green and deep yellow - These are called "Fancy diamonds". In the United States and around the world colorless diamonds are graded on an alphabetical scale, introduced by the Gemological Institute of America (GIA). "Colorless" or "rare white" diamonds are of color grades D, E and F. Diamonds of color grade D are very rare, and extremely valuable.

Diamond Carat Weight

The weight of a diamond is measured in carats (ct.). Each carat is divided into 100 points. For example:  1ct.= 100 points, 1/2 ct.  = 50 points, etc. The carat weight alone is almost meaningless unless you also  consider the cut, clarity and color of the diamond.  A large diamond is not very valuable if it lacks brilliance,  purity and high-grade color. However, since larger stones are rarer than smaller ones, diamond value rises  exponentially with carat weight. Therefore, a diamond weighing 3.0 carats, will always be worth more than  three 1.0 carat stones of the same quality. No two diamonds are  exactly alike, and you must weigh all of  the factors - color, cut, clarity and carat weight - when making your diamond jewelry buying decision.