Certain diamond enhancement techniques are commonly used to artificially produce an array of colors, including blue, green, yellow, red, and black. Color enhancement techniques usually involve irradiation, including proton bombardment via cyclotrons; neutron bombardment in the piles of nuclear reactors; and electron bombardment by Van de Graaff generators. These high-energy particles physically alter the diamond's crystal lattice, knocking carbon atoms out of place and producing color centers. The depth of color penetration depends on the technique and its duration, and in some cases the diamond may be left radioactive to some degree.