Diamonds have remarkable optical characteristics. Because of their extremely rigid lattice, they can be contaminated by very few types of impurities, such as boron and nitrogen. Combined with wide transparency the results in the clear, colorless appearance of most natural diamonds. Small amounts of defects or impurities, about one per million of lattice atoms, color diamond blue (boron), yellow (nitrogen), brown (lattice defects), green (radiation exposure), purple, pink, orange or red, uncut diamonds.