Rudraksha, also rudraksh, Sanskrit: rudrākṣa ("Rudra's eyes"), is a seed is traditionally used for prayer beads in Hinduism and Buddhism. The seed is produced by several species of large evergreen broad-leaved tree in the genus Elaeocarpus, with Elaeocarpus ganitrus being the principal species used in the making of organic jewellery or mala.
Rudraksha, being organic, is preferentially worn without contact with metal; thus on a cord or thong rather than a chain.
Examples:
Source:http://en.wikipedia.