Raiohalos are microscopic, spherical shells of discolouration in rocks, such as granite, or biotite caused by the inclusion of radioactive grains in the rock or by deposition of radioactive material in them. One widely accepted theory is that the discolouration is caused by alpha particles emitted by the nuclei; the radius of the concentric shells are proportional to the particle's energy (Henderson & Bateson 1934). The phenomenon of radiohalos has been known to geologists since the early part of the 20th century, but wider interest was prompted by the claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth (Gentry 1992). The claims are contested by the mainstream scientific community as an example of creationist pseudoscience (Wakefield 1988).