Rarely is something more valuable when it is lacking something, but when considering a (white) diamond, the absence of color is something to strive for. The more clear or white a diamond is, the more valuable it is.
So what causes this yellow tint? The presence of Nitrogen in a diamonds atomic structure when it is being formed can cause a diamond to emerge with a yellow tint.
Below is the accepted color scale for GIA (Gemological Institute of America), IGI (International Gemological Institute), EGL (European Gemological Laboratory) and other diamond grading agencies.
This chart was created by GIA in the 1950’s and is used to measure the lack of yellow in “white” diamonds. With D graded diamonds being completely Colorless, all the way to Z indicating a Light Yellow tint.