Master Stones
mas·ter stones | ˈma-stər stōnz
noun
A set of diamonds used as a key to grade the color of other diamonds
A master stone is a type of diamond that is used by jewelers to define the color of another diamond during the grading process. Because color grading is such a delicate process and there are no strict scientific means by which to do so, master stones are used as points of comparison to provide accurate gradings based on the true color of a diamond when compared to one considered to be a certain grade. These two stones will be compared to one another on a pure white surface with ordinary indoor lighting to ensure real-world conditions. For a diamond to be used as a master stone, it must have an SI clarity level, weight 70 points or more, and match specific proportions based on the grading scale being used.