I have looked at your picture several times and have some thoughts. The pendants are beautifully carved and of a quality that is not found in recent examples. What Rosanna thinks is mud I think is a mixture of sweat and dirt accumulated during years of use. I have seen this type of deposit on uncleaned, old, well used beads many times in the past but not recently. The material used for stringing is not usually used in modern examples. Also the addition of a few white hearts seems whimsical not the structured arrangement of modern arrangements. I feel there is more than a good possibility that the necklace is old and authentic. Without seeing it in reality I don't feel comfortable making a definite analysis but I think you should widen your research. I've attached a picture of some soiled beads from the Dinka tribe and a picture of some ivory pendants from the Songye tribe, Congo. the pictures were taken from Angela Fisher's book Africa Adorned.
good luck Judy