Hijiki is a brown, stringy seaweed growing in coastal waters around Japan. It dries black so you have these black noodle like threads of seaweed that are rich in minerals. It is traditionally used in small amounts by simmering until tender then being added to other dishes including salads. There is a CAUTION with hijiki; it does contain small amounts of arsenic, more so than any other seaweed. I have used it in the past with no ill effect. Also see Seaweed for a complete list of sea vegetables.