I have a strand of black color pearl necklace . Is there any way that I can tell if their color are natural black color or dyed black color.
It depends on the pearl type. If your black color pearl necklace is not made of Tahitian pearl, I guess, most likely, your black color pearls are treated-color, not natural black color. Almost all the black color Akoya and freshwater pearls in the market are treated colors.
It is well known that genuine Tahitian pearls have natural black color. Tahitian pearls are cultured in the lagoons of French Polynesia and recently in some new locations of Australia, for an instance, Western Australia has been producing pearls from Pinctada Margaratifera Oysters for some years now. (Thanks to Jim & Lou from W.A.Rainbow Pearls & Shell Products,to provide this information.)
Color treatment are one of the steps of Post-harvest processing for Akoya and freshwater pearls. It's very common that pearl farmer process their pearls after harvest in one way or another. However, treatment is not a bad thing, it can create affordable alternative pearls in a full range of exotic colors to their natural-colored counterparts and offer customer affordable pearls in a variety of bright hues to complement their wardrobes.
Please keep this in mind, if you are purchasing black color cultured freshwater or Akoya saltwater pearls, the black color are most likely NOT natural black color.