Irradiated Quartz Crystal (Smoky Quartz)

The photograph shows two quartz crystals each of which is approximately 1.5" across. The difference between the two is that the crystal on the right has been irradiated with cobalt-60 gamma rays to turn it gray. Gray-colored quartz is referred to as "Smoky Quartz." The color of  the smoky quartz found in nature is due to its exposure to gamma rays from naturally occurring radionuclides such as potassium-40 and members of the uranium and thorium decay series. For the quartz to turn gray, it must contain trace amounts of aluminum.

References

• Radiation, Heat - Improve Natures Gems. Lithosphere, June 1993.
• Ashbaugh, C.E. Radioactive and Radiation Treated Gemstones. Radioactivity and Radiochemistry, Vol. 2(1): 42-56.