The 4C’s that should be considered while buying loose diamond are:

1. Color - Color decides the amount of yellow touch in the diamond

2. Cut - Cut determines how the light is refracted from the diamond.

3. Clarity - Clarity determines the impurities or flaws in the diamond

4. Carat - Carat is the weight of the diamond 

The noteworthy site for loose diamonds is http://www.b2cjewels.com/loosediamonds.aspx

It also provide interactive search which helps in the search of diamonds according to their characteristics and price

http://www.b2cjewels.com/interactivesearchresult.aspx