Diamonds are formed from carbon under great heat and pressure. Normally they have no colour but they can take on different hues under certain circumstances. This is rare, and coloured diamonds are so far more valuable than standard ones. Green diamonds are the rarest colour of all and so the most costly.
Different tints usually come from various impurities being incorporated into the stone when it forms. Green diamonds are different. Their colour comes from exposure to radiation. In nature if diamonds are exposure to naturally occurring radiation, usually from minerals such as uranium, during formation they will take on a subtle green tint.
The effect of exposing a crystal such as a diamond to radiation is to permanently change its structure. The colour in green diamonds comes not from added pigmentation but from the internal structure of the stone being changed.
The shades green diamonds are found in range from sea green, to near yellow, with others showing brown and blue tones. None looks like an emerald, the tinting is slighter and far more subtle.
In the laboratory it is easy to create green diamonds but in nature the right combination of circumstances is a very uncommon occurrence. This means that jewellers are often unwilling to verify that a green diamond is a natural one, given how valuable these are.
Naturally occurring green diamonds probably encounter radiation during formation. In the laboratory, the irradiation obviously is applied later. These differences can be seen in the diamonds later. Tiny inconsistencies in the colour and an absence of absorption lines can indicate the colour is natural but are not definitive proof.
The most famous green diamond, whose colour without a doubt is natural, is the Dresden diamond. The Dresden diamond was discovered and cut in the 18th century. It was probably discovered in India, and initially valued at £10,000, which was an enormous sum at the time.
The Dresden diamond is a large stone, weighing in at 41 carats. Its colour is unusual, not just for being green but also for being remarkably even. This is also an unusually well cut diamond for the time. It was examined carefully in 1988 and still considered to be the largest and finest green diamond in the world.
Unless you are very rich indeed, and prepared to spend huge sums on stones, you are unlikely to own a natural green diamond yourself. However the delicate, watery green is reproduced very convincingly not only in diamonds that have been made green artificially but also in all kinds of much cheaper imitation materials. Very precious stones are probably best admired in museums but just about anybody can enjoy owning a copy if they so wish.