Is the buying or wearing of birthstones considered to Avoda Zara? In particular having several birthstones on one necklace to represent the birthstone of several people?
My apologies, but I am not familiar enough with this topic to give you an answer if it is avoda zara or not. The only thing I can say about it, is that it is not an accepted practice among Torah Jews, and it doesn’t have a Jewish source. Therefore I would be wary of it.