The living heart of Nuba religion is the kujur, the diviner-priest into whom a spirit descends, and the power that possesses him is the being here registered. In trance the spirit speaks through its medium to uncover the concealed causes of sickness, death, and misfortune, to accuse witches, to prescribe cure and expiation, and, in the hands of the greatest practitioners, to make rain. What most sharply distinguishes the Nuba case, and what drew Nadel to call it shamanism, is the role of small green stones: the spirit is materially lodged in these stones, which the diviner keeps, handles, and passes to a successor, so that the tie between spirit and medium is inheritable through the objects. Sources vary on the spirit's nature, sometimes casting it as an ancestral shade, sometimes as a free spirit of the hills or waters, and the boundary between the diviner's power, the ancestors, and the rain is not sharply drawn.