rare canary diamonds belong to this type, which represents only 10% of known natural diamonds. Synthetic diamond containing nitrogen is Type Ib. Type I diamonds absorb in both the infrared and ultraviolet region, from . They also have a characteristic fluorescence and visible absorption spectrum (see Optical properties of diamond). Type II 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 , unlike Type I diamonds. 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 (1.8% of gem diamonds), 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 (see Electrical properties of diamond). 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 caused by GR1 color centers in the crystal lattice produced by exposure to varying quantities of radiation. Pink and red are caused by plastic deformation of the crystal lattice from temperature and pressure. Black diamonds are caused by microscopic black or gray inclusions of other materials such as graphite or sulfides and/or microscopic fractures. Opaque or opalescent white diamonds are also caused by microscopic inclusions. Purple diamonds are caused by a combination of crystal lattice distortion and high hydrogen content. Colorless diamonds The majority of mined diamonds fall between white and pale yellow or brown; what is known as the normal color range. Diamonds of more intense color (usually yellow, but in some cases red, green or blue) are termed fancy color diamonds. Black diamonds are also fancy color diamonds. All other factors being equal, the most valuable diamonds are those with the least color (appearing white to the eye), and those possessing vivid color ( especially yellow, pink, and blue). Diamonds at the low end of the normal color range, which possess a pale yellow or brown hue, are the least desirable for jewelry. Color is one of the four traditional factors by which a diamond is evaluated (the others being carat weight, clarity, and cut).