Rough greenish diamonds tend to occur in one of three forms: a stone, frequently a crystal, that bears a light green tinge similar to that of a swimming pool full of water; a stone with a "skin" color of dark green; or a yellowish-green stone that is slippery to some degree.

The first two types of green diamonds mentioned tend to lose their greenish color after the cutting and polishing process and are classed as white diamonds or "silvery capes" – diamonds with a light yellow hue.

The third, yellowish-green type of stone accounts for true green diamonds – only a few of which are known to exist. De Beers Fancy Colored Diamonds collection includes some stunning example of these rare stones.

One of the best-known green diamonds is the Dresden Green, named after the capitol of Saxony. The first reference to the Dresden Green diamond appeared in a London newspaper in 1722.
The Dresden Green has a long history, and was displayed at the Smithsonian Institution until January 2001, when it was returned to the Albertinium Museum in Dresden.

What causes diamonds to take on a green color? While other colored diamonds are the result of exposure to specific elements (boron or nitrogen) during the formation process, green diamonds are the result of irradiation – exposure over millions of years to some form of radioactivity.

The most common form of diamond irradiation occurs when alpha particles present in uranium compounds or groundwater bombard the diamond crystal, causing a green spot to form on the diamond's surface. If the irradiation includes beta and gamma particles, the green color will penetrate the diamond, turning the entire stone green under the proper circumstances.

The color change itself is attributable to the particle bombardment causing a change in the diamond crystal's lattice structure. 
A green diamond's color can be enhanced through heating, but the temperature must be kept below 600°C, at which point it can turn the color of the diamond into yellow or brown.

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