All about diamonds

In general thought, the colour of a diamond is not directly related to the
impurities and their concentrations but it due to defects produced in the
lattice by the impurity atom. Bombardment of the crystal by atomic particles
can knock out carbon atoms from their positions in the crystal lattice,
produce defects and so induce colour. Gamma rays and X-rays do not have any
mass and so cannot displace the carbon atoms and thus do not produce
defects.

Exposed to a stream of high energy electrons, the diamonds attain a blue
shade, the depth depending on the penetration of the electron into the
stone. Electron -hole pairs are trapped at the defects that absorb red light
and so colour the stone blue, but this colour fades with gentle heating. A
high energy electron colliding with a carbon atom could give it enough
energy for it to knock other carbon atoms out of their places. For example,
electrons of 13 Me V from a big accelerator can give a carbon atom a maximum
energy of 35 ke V; this atom is capable of displacing about a hundred other
atoms. This cascade of collosions tends to make the diamond greenish in
colour. Neutrons and alpha particles are far more effective in tranferring
energy to the carbon atoms and the cascade of collisions turn the diamonds
into a definite green colour. Even the highest energy alpha particles from a
cyclotron penetrate the diamond only a small distance and the colour is a
surface effect that may disappear when the stone is polished.