<P> 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 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 , from 320 nm ( 3.2 × 10 m ) . They also have a characteristic fluorescence and visible absorption spectrum ( see Optical properties of diamond ) . </P>
yellow colour in diamonds is caused by the presence of
nitrogen atoms are dispersed throughout the crystal in isolated sites