Irradiation intensifies or changes a diamond’s color and is the only diamond treatment that exists in nature as well as in laboratory conditions.
The first experiments that used irradiation as a method of diamond changing the color of diamonds were conducted by Sir William Crookes in 1904. He was able to turned a diamond dark green, but the stone became so radioactive that it was unwearable! Glad they don't use his way today!
Current irradiation methods are safe and produce green, blue, or black colored diamonds, but by increasing the mobility of the stone’s carbon atoms – a process known as annealing – irradiated diamonds can be turned yellow, orange, brown, or pink as well. Two methods of changing a diamond’s color through irradiation are noted below:
Cyclotroning: A diamond is bombarded with protons and deuterons via cyclotrons. Cyclotroned diamonds take on a superficial blue-green or green color and are annealed to 800°C to turn them yellow or orange. This method is quite rare now.
Gamma rays: This is the cheapest and safest method of irradiating diamonds and quite common in the industry, but the treatment usually takes months which also drives up the costs
. Gamma ray bombardment of a diamond through exposure to cobalt-60 produces a blue to bluish-green color that penetrates the entire stone.
You will note that often jewelers post if the diamonds are natural or irradiated (IR). This is to help you understand the value of what you are purchasing as natural stones do tend to sell at a premium.