A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or color. However, in reality almost no gem-sized natural diamonds are absolutely perfect. The color of a diamond may be affected by chemical impurities and/or structural defects in the crystal lattice. Depending on the hue and intensity of a diamond's coloration, a diamond's color can either detract from or enhance its value. For example, most white diamonds are discounted in price when more yellow hue is detectable, while intense pink diamonds or blue diamonds (such as the Hope Diamond) can be dramatically more valuable. Of all colored diamonds, red diamonds are the rarest. The Aurora Pyramid of Hope displays a spectacular array of naturally colored diamonds, including red diamonds. History of color grading Color grading of diamonds was performed as a step of sorting rough diamonds for sale by the London Diamond Syndicate. As the diamond trade developed, early diamond grades were introduced. Without any co-operative development, these early grading systems lacked standard nomenclature and consistency. Some early grading scales were; I, II, III; A, AA, AAA; A, B, C. Numerous terms developed to describe diamonds of particular colors: golconda, river, jagers, cape, blue white, fine white, gem blue, brown, etc. Possible colors Diamonds occur in a variety of colors—steel gray, white, blue, yellow, orange, red, green, pink to purple, brown, and black. Colored diamonds contain interstitial impurities or structural defects that cause the coloration; pure diamonds are perfectly transparent and colorless. Diamonds are scientifically classed into two main types and several subtypes, according to the nature of impurities present and how these impurities affect light absorption. Type I Type I diamonds have nitrogen atoms as the main impurity, commonly at a concentration of 0.1%. If the nitrogen atoms are in pairs they do not affect the diamond's color; these are Type IaA. If the nitrogen atoms are in large even-numbered aggregates they impart a yellow to brown tint (Type IaB). About 98% of gem diamonds are type Ia, and most of these are a mixture of IaA and IaB material: these diamonds belong to the Cape series, named after the diamond-rich region formerly known as Cape Province in South Africa, whose deposits are largely Type Ia. If the nitrogen atoms are dispersed throughout the crystal in isolated sites (not paired or grouped), they give the stone an intense yellow or occasionally brown tint (Type Ib); the