These color stones range in intensity from faint to intense green. Pure green stones are extremely rare, and most natural green stones include one or even two secondary hues, typically yellows, blues, browns and grays of different intensities. The source of the green color exposure over millions of years underground to radioactive minerals, such as uranium, even though diamonds themselves are not radioactive. Radiation occurring in nature is similar to the type of radiation used to color treat diamonds, but natural green diamonds tend to display green or brown surface spots, while in treated green diamonds the color is spread homogenously throughout the stone.