The best clay for aromatherapy necklaces is typically porous and absorbent, allowing it to effectively hold and release the essential oils. One commonly used clay for this purpose is terracotta clay, which has natural pores that can soak up the oils. Another option is white or beige earthenware clay, which tends to have a similar absorbency. Both of these clays are often chosen for their ability to retain the scent of the essential oils and provide a prolonged aromatherapy experience.