In the rough, greenish diamonds tend to occur as one of three types: a stone, often a crystal shape, possessing a light tinge rather like the color of water in a swimming pool; a stone with a dark green skin; a yellowish-green stone characterized by a degree if lubricity. After being cut and polished, diamonds of the first and second types usually lose their greenish color to become white gems or, alternatively, light yellow stones known as "silvery capes". The few truly green faceted diamonds therefore originate from the third type. The famous collection of De Beers Fancy Colored Diamonds, which has been displayed throughtout the world includes some beautiful examples of green diamonds.
Since this is the story of a truly rare gem, a scientific explanation for the phenomenom of green diamonds is needed. The green color is usually caused by the crystal's coming into contact with a radioactive source at some point during its lifetime, and in geological terms, this is measured in millions of years. The most common form of irradiation diamonds chance into is through bombardment by alpha particles which are present in uranium compounds or percolating groundwater. Long exposure to these particles forms a green spot on the surface of the diamond, or sometimes produces a thin green coating which is only skin deep and can easily be removed during the faceting process. But bombardment by beta and gamma rays well as neutrons will color the stone to a greater depth and in some cases turn the whole stone's interior green.
Heating the stone might sometimes improve the color but care must be taken to keep the temperature below 600°C, because at this temperature the green color is likely to turn to a light yellow or brown. The change in color is caused by the change in the crystal's lattice structure. Before bombardment by radioactive particles the crystal's lattice was stable but the initial radioactive shock was sufficient to disturb the equilibrium and produce a green coloration. Tempering will distort the lattice further abd produce another change of color. This phenomena is analogous to a piece of elastic that has been overstretched; it will stretch back so far, but never returns to its original length. Similarly, after a treatment the diamond's lattice remains permanently distorted.
Research has disclosed that green or irradiated diamonds are more common from alluvial deposits, although they are found in primary sources, usually in the upper part of the diamond-bearing volanic pipe, but green diamonds of any size are rare. The Dresden Green, which probably weighed over 100 (old) carats in its rough form, is unique amoung world famous diamonds. It was originally probably an elongated unbroken stone since greenish diamonds rarely occur as cleavages.
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. The issue dated October 25th - 27th, 1722 included this article:
"On Tuesday last, in the afternoon, one Mr. Marcus Moses, lately arrived from India, had the honor to wait on his Majesty [King George I (ruled 1714-27)] with his large diamond, which is of a fine emerald green colour, and was with his Majesty near an hour. His Majesty was very much pleased with the sight thereof. It is said there never was seen the like in Europe before, being free from any defect in the world; and he has shown his Majesty several other fine large diamonds, the like of which 'tis said were never brought from India before. He was also, the 25th, to wait on their Royal Highnesses with his large diamond; and they were surprised to see one of such largeness, and of such a fine emerald color without the help of a foil under it. We hear the gentlemen values it at £10,000."
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