A natural diamond coming into contact with a radioactive source at some point during its lifetime causes some diamonds to develop a green coloration. The time required may be as much as a million years or longer. Green diamonds of this nature are very unique.
The most common form of irradiation diamonds comes from alpha particles found in uranium compounds or from percolating groundwater. Green spots on the surface of the diamonds or a thin green film may develop on the skin of the diamond after long exposure to these particles. Many times this green coloration will be removed during the cutting or faceting process.
Bombardment by beta and gamma rays will color the diamond to a greater depth and in some rare case turn the entire stone green. Heating to temperature to just below 600 degrees Celsius can sometimes also cause a diamond to develop a green ting. Higher temperature may turn the stone to a less desirable yellow or brown color.
The diamonds crystal's lattice structure is changed by the bombardment by radioactive particles enough to disturb the equilibrium of the crystal's structure and produce a green coloration. Addition heating will distort the lattice further and add or create another color change. Both radiation and heating will permanently distort the diamonds structure.
Some lightly colored diamonds are irradiated to make their color more intense. This means that low fields of radiation are beamed into the cut and polished stone, darkening the outer part of the stone all the way around. The process is permanent and professionally accepted in the diamond industry.
Probably the largest irradiated diamond is the Deepdene, a 104.88-carat golden yellow cushion shaped stone. The diamond was sold in Switzerland in 1997 for $715,320.
The highest price ever paid per carat for a colored diamond was $926.315 for a 0.95 carat fancy Purplish Red stone sold at Christies Auction House in 1987. A vivid green brilliant at .90 carat was sold for $670,00 per carat in 2000.
The largest natural green diamond in the world is The Dresden Green . It is classified as an "apple-green" color and weighs in at 40.70 carats. The Gemmological Institute of America examined the stone in 1988. The Dresden Green Diamond was proved to be not only of extraordinary quality, but also a rare type IIa diamond and is consider the largest and finest natural green diamond ever found. The Dresden Green gets its name from the capitol of Saxony where it has been on display for more than 200 years. The earliest known reference to its existence occurs in The Post Boy, a London new-sheet of the 1700's.