The degree of colorlessness is typically used to assess the colour grade of gem-quality diamonds. Diamonds that are chemically pure and structurally perfect are colourless and have a higher value. The GIA colour level reliably tests colorlessness by contrasting it to colour samples of each colour grade under particular lighting conditions.
Color variations between diamonds can be slight and undetectable to the untrained eye, but these differences have a significant impact on the appearance and price of diamonds. The GIA colour rating system for determining the colour of diamonds is the most widely used in the global gem industry.
Diamond's initial "D" reflects the highest level of transparency. The lower the rating, the yellower the product.