[Wood Ear, Cloud Ear, Tree Ear, Mouse ear, Jelly mushroom; Kikurage (Japan); Mu-ehr (China); Auricularia polytricha, also A. auricula-judae (Jew ear), Hirneola auricula-judae and others (variation in color)]
This thin nearly black fungus is sold dried and will expand to about 4 times the dried volume and about 5 times the weight when soaked. It is often used in soups and stir fries in China and Korea for its slippery but crunchy texture. It has little flavor of its own but does absorb flavors from other ingredients it's cooked with. In the photo there are three dried ones on the left, each a little over and inch across, and on the right a rehydrated one that was originally the same size as one of the others.
It is held in Chinese medicine to improve blood circulation and relieve
atherosclerosis. Preliminary tests in Western medicine are encouraging and
include confirmation of anticoagulant properties.
Buying: Black fungus can be found in just about any East or Southeast Asian market packed in cellophane bags. Not much to buying them except bigger is better (easier to handle). It is sometimes available fresh in shrink wrapped trays at markets in Los Angeles and other areas with large Asian populations.
Technically, "cloud ear" and "wood ear" are supposed to be different sizes / varieties, but that and any other distinctions are pretty much lost in commercial products. Most available in California is just called "black fungus".
Preparation: Place dried fungus in a heat proof bowl and pour over enough hot water to cover after expansion (they will expand a lot). Let soak for 30 minutes or more, then rinse. Remove any woody anchor points and tear to sizes that match other ingredients.