Diamonds occur in a variety of colors — steel gray, white, blue, yellow, orange, red, green, pink to purple, brown, and black, virtually in every color of the rainbow. Colored diamonds contain interstitial impurities or structural defects that cause the coloration, whilst pure diamonds are perfectly transparent and colorless.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 or blue diamonds can be dramatically more valuable. Out 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. Diamonds occur in a variety of colors — steel gray, white, blue, yellow, orange, red, green, pink to purple, brown, and black, virtually in every color of the rainbow. Colored diamonds contain interstitial impurities or structural defects that cause the coloration, whilst pure diamonds are perfectly transparent and colorless. Diamonds are of two main types and several subtypes, according to the nature of impurities present and how these impurities affect light absorption. 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. 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. If the nitrogen atoms are dispersed throughout the crystal in isolated sites, they give the stone an intense yellow or occasionally brown tint of Type Ib; the rare canary diamonds belong to this type, which represents only 0.1% of known natural diamonds. Synthetic diamond containing nitrogen is Type Ib. Type I diamonds absorb in both the infrared and ultraviolet region. They also have a characteristic fluorescence and visible absorption spectrum. Type II diamonds have no measurable nitrogen impurities. Type II diamonds absorb in a different region of the infrared, and transmit in the ultraviolet below 225 NMI. They also have differing fluorescence characteristics, but no discernible visible absorption spectrum. Type IIa diamond can be colored pink, red, or brown due to structural anomalies arising through plastic deformation during crystal growth—these diamonds are rare, but constitute a large percentage of Australian production. Type IIb diamonds, which account for 0.1% of gem diamonds, are usually light blue due to scattered boron within the crystal matrix; these diamonds are also semiconductors, unlike other diamond types. However, a blue-grey color may also occur in Type Ia diamonds and be unrelated to boron. Also not restricted to type are green diamonds, whose color is from exposure to varying quantities of radiation.