All polished diamonds are valuable. That value is based on a combination of factors. Rarity is one of those factors. Diamonds with certain qualities are more rareâ€”and more valuableâ€”than diamonds that lack them. Diamond professionals use the grading system developed by the Gemological Institute of America in the 1950s, which established the use of four important factors to describe and classify diamonds: Clarity, Color, Cut, and Carat Weight.