Nori by John Spottiswood.

These thin dark sheets are used to make sushi. They're usually a dark purplish-black, but they turn green and acquire a pleasant, nutty flavor when toasted. You can make your own toasted nori sheets by passing nori sheets over a flame a few times. Yaki means cooked in Japanese, so pretoasted nori sheets are labeled yaki-nori or yaki sushi nori. Look for toasted and untoasted sushi nori in the Asian foods section of large supermarkets. The name nori is also used for laver, the plant that sushi nori is made from. Unlike sushi nori, laver should be rehydrated before use. If you can't find sushi nori, one option is to make sushi without a wrapper. It helps to use plastic wrap to shape the roll.