An important value factor to consider when grading pearls is the colour - both, overtone and body colour.
Pearls (Perlen) come in a wide variety of colours, ranging from white all the way to black. Here again (as with many of the characteristics of pearls), the fact that the pearl is an organic gemstone, formed within a living creature, contributes to the myriad unique ways in which its colouration can develop.
The natural colour of a pearl results from a combination of several factors:
The pearl's bodycoulor is its main colour. This can be white, silver, cream-colored, gold, green, blue, or even black. The bodycoulor is determined by the type of oyster or mollusc that produces the pearl (certain types of oysters generally produce pearls of certain colours), as well as the conditions of the water and sometimes the type of nucleus which is implanted to stimulate the pearl's creation.
Overtones are translucent colours which sometimes appear over top of the pearl's main bodycoulor. These overtones tend to alter the bodycoulor somewhat, as well as adding depth and glow. A pearl may be white with rose overtones, for example. Some pearls have no overtones at all.
The term orient refers to the shimmering, iridescent colours which appear to move and glitter when the pearl is turned. This phenomenon is caused by the way in which light is reflected through the various thin layers of nacre,which make up the pearl.
It is important to note that many pearls are artificially coloured. This is widely practiced with freshwater, Akoya and at times Black tahitian pearls. The colours are artificially infused by a treatment known as dying or by subjecting the pearls to irradiation. These treated colours are typically easy to spot by a trained observer who may peer down the drill hole looking for concentrations of colour which indicates the presence of dye or a darkened pearl nucleus which indicates radiation treatment.
One rule of thumb in spotting treated pearls is that if the pearls are not true tahitian pearls, from the Black Lip Oyster (Pinctada margaritifera), they cannot naturally be black. Naturally black akoya pearls and freshwater pearls do not exist and if black pearls of this variety are offered for sale they will always be dyed. Many unwary consumers buy black pearls-only to find out later that the natural colour grading is false.
In the 1930s freshwater pearls from Japan's Lake Biwa introduced a wide variety of new colours to the pearl market -- colours which were previously unavailable in saltwater pearls. Today chinese freshwater pearl farmers have continued this trend with many fancy coloured pearls harvested in large numbers.