Iwa Nori is a variety of seaweed in Japan that grows in rock crevices along the shore.
It was traditionally, of course, just harvested from the wild, but now it is cultivated in nets.
It is dried, then rehydrated for use in soups, one-pot dishes, and particularly in dishes that have some vinegar in them. While it can be used for sushi, Asakusa Nori is generally preferred.
Language Notes
The name in Japanese means “rock seaweed”.