Cap 4-12 mm broad, convex, expanding to nearly plane, sometimes centrally depressed to infundibuliform; margin decurved, then level, occasionally wavy; surface moist, not viscid, pellucid-striate, pruinose when young (use hand lens), dark brown at the disc, pale-tan towards the margin; context thin, < 1 mm, watery light-brown, unchanging; odor and taste mild.
Gills decurrent, fairly well-spaced, at first whitish, then cream-colored, edges even; lamellulae in up to three series.
Stipe 1.5-4.0 cm long, approximately 1 mm thick, round, equal except enlarged at the apex; surface like the cap, pruinose, especially at the apex, becoming nearly glabrous with age and handling; color dark vinaceous-brown to bluish-black at the apex, pale buff-orange below; partial veil absent.
Spores 4.0-5.5 (6) x 2.5-3.5 µm, elliptical in face-view, similar in profile but slightly inequilateral, smooth, thin-walled, contents granular, hilar appendage well developed, inamyloid; spores whitish in deposit; angular crystals present in mounts of gill tissue.
Scattered to gregarious in moist, shaded locations, typically in moss beds; known from coastal forests in the fall; rare.
Small and easily overlooked, Rickenella swartzii is nonetheless distinctive with a two-toned cap that varies from dark-brown to deep purple-brown at the disc to cream-colored at the margin. Its other fieldmarks are characteristic of the genus: fruiting with and possibly parasitizing mosses, a finely pubescent cap, strongly decurrent gills, and a relatively long, thin stipe. A more commonly collected cousin, Rickenella fibula has a yellowish-orange cap and stipe. Rickenella swartzii is most likely to be confused with Lichenomphalia umbellifera, commonly known as Omphalina ericetorum. This basidiomycete lichen (algae occur within its tissues) has a brownish to yellowish-brown, striate cap, but lacks the strong contrasting colors and finely pubescent surface of Rickenella swartzii. It also differs in its fruiting habit, typically on algae-covered rotting wood and soil. Compare also with Mycena species which can usually be distinguished by adnate to adnexed gills.