Cap 1.5-4.0 cm broad, rounded, becoming campanulate (bell- shaped), finally convex to nearly plane, typically with a broad umbo; margin striate when moist, sometimes upturned in age; surface dry, smooth, light cinnamon-brown to buff-brown when young, fading in age to pale-buff or cream; flesh thick, reviving after being dried; odor and taste mild.
Gills adnexed to nearly free, subdistant, interspersed with shorter lamellae and intervenose; cream to buff-colored.
Stipe 2-6 cm tall, 2-5 mm thick, equal, tough, cream to buff above, the base usually somewhat darker and pruinose; veil absent.
Spore 7-8.5 x 4-5.5 µm, elliptical, inequalateral, apiculate (pointed at the end), smooth, nonamyloid; spore print white to pale-cream.
Forming arcs or rings in lawns and pastures, spring, summer and fall.
Edible and excellent, but the tough, fibrous stipes should be discarded. Caution is advised as there are toxic lookalikes (see below).
Marasmius oreades is distinguished by a smooth, cream to buff-colored, umbonate cap, well spaced, adnexed to nearly free gills, tough, wiry stipe, and white spores. It get its common name from the distinctive circular fruitings it forms in grassy areas. Fairy rings of course are not unique to Marasmius oreades. In our area, other common species which form rings or arcs include Clitocybe nuda (Blewit), Agaricus campestris (Field Mushroom), A. arvensis (Horse Mushroom), Stropharia ambigua and several species of coral mushrooms. Like many Marasmius species, dried specimens of the Fairy Ring mushroom have the unusual ability to revive when moistened. A toxic species that somewhat resembles Marasmius oreades, and which can also form fairy rings, is Clitocybe dealbata. It has a white to greyish-white, umbo-less cap, closely-spaced, slightly decurrent gills, but is uncommon in the S. F. Bay Area. Some species of Inocybe could also be mistaken for the Fairy Ring mushroom, but all have brown spores and if found in grass are usually near trees.