About Yellow Diamonds:

Yellow diamonds were once thought to be inferior.  Paler yellows, such as canary yellow, are more common but are not as desirable as their darker counterparts. Fancy vivid Yellow diamonds are the most rare and valuable of the yellow diamonds.

Color Orogin:

Yellow diamonds are colored because of the impurities that are trapped inside diamonds when they are created.  If a few of the millions of carbon atoms have been replaced by nitrogen atoms, then structure of the diamond will not be significantly altered but the clarity will be changed.  The amount of color displayed is dependent on the amount of nitrogen involved.

Yellow diamonds are rarely found in diamond mines. Only estimated 1/10 of 1 percent of diamonds mined have an intense yellow color.