Coprinellus micaceus, commonly known as the mica cap, glistening inky cap, or shiny cap, is a common species of mushroom-forming fungus in the family Psathyrellaceae.

Formerly known as Coprinus micaceus, the species was transferred to Coprinellus in 2001 as phylogenetic analyses provided the impetus for a reorganization of the many species formerly grouped together in the genus Coprinus. Based on external appearance, C. micaceus is virtually indistinguishable from C. truncorum, and it has been suggested that many reported collections of the former may be of the latter.

Depending on their stage of development, the tawny-brown mushroom caps may range in shape from oval to bell-shaped to convex, and reach diameters up to 3 cm (1+1⁄4 in). The caps, marked with fine radial or linear grooves that extend nearly to the center, rest atop whitish stipes up to 10 cm (4 in) long. In young specimens, the entire cap surface is coated with a fine layer of reflective mica-like cells. Although small and with thin flesh, the mushrooms are usually bountiful, as they typically grow in dense clusters. A few hours after collection, the gills will begin to slowly dissolve into a black, inky, spore-laden liquid—an enzymatic process called autodigestion or deliquescence.

With a cosmopolitan distribution, the saprobe typically produces clusters on or near rotting hardwood tree stumps or underground tree roots. The fruit bodies are edible before the gills blacken and dissolve; cooking stops the autodigestion process. Chemical analysis of the fruit bodies has revealed the presence of antibacterial and enzyme-inhibiting compounds.

Coprinellus micaceus was illustrated in a woodcut by the 16th-century botanist Carolus Clusius in what is arguably the first published monograph on fungi, the 1601 Rariorum plantarum historia (History of rare plants), in an appendix, Clusius erroneously believed the species to be poisonous, and classified it as a genus of Fungi perniciales (harmful fungi). The species was first described scientifically by French botanist Jean Baptiste François Pierre Bulliard in 1786 as Agaricus micaceus in his work Herbier de la France. In 1801, Christian Hendrik Persoon grouped together all of the gilled fungi that auto-digested (deliquesced) during spore discharge into the section Coprinus of the genus Agaricus. Elias Magnus Fries later raised Persoon's section Coprinus to genus rank in his Epicrisis Systematis Mycologici, and the species became known as Coprinus micaceus. It was the type species of subsection Exannulati in section Micacei of the genus Coprinus, a grouping of related taxa with veils made of sphaerocysts (round swollen cells usually formed in clusters) exclusively or with thin-filamentous connective hyphae intermixed. Molecular studies published in the 1990s demonstrated that many of the coprinoid (Coprinus-like) mushrooms were in fact unrelated to each other. This culminated in a 2001 revision of the genus Coprinus, which was split into four genera; C. micaeus was transferred to Coprinellus.

Due partly to their ready availability and the ease with which they may be grown in the laboratory, C. micaceus and other coprinoid mushrooms were common subjects in cytological studies of the 19th and 20th centuries. The German botanist Johann Heinrich Friedrich Link reported his observations of the structure of the hymenium (the fertile spore-bearing surface) in 1809, but misinterpreted what he had seen. Link thought that microscopic structures known today as basidia were thecae, comparable in form to the asci of the Ascomycetes, and that each theca contained four series of spores. His inaccurate drawings of the hymenium of C. micaceus were copied in subsequent mycological publications by other authors, and it was not until microscopy had advanced that mycologists were able to determine the true nature of the basidia, when nearly three decades later in 1837 Joseph-Henri Léveillé and August Corda independently published correct descriptions of the structure of the hymenium. In 1924, A. H. Reginald Buller published a comprehensive description and analysis of the processes of spore production and release in the third volume of his Researches on Fungi.

The specific epithet micaceus is derived from the Latin word mica, for "crumb, grain of salt" and the suffix -aceus, "like, similar"; the modern application of "mica" to a very different substance comes from the influence of micare, "glitter". The mushroom is commonly known as the "shiny cap", the "mica cap" or the "glistening inky cap", all in reference to the mealy particles found on the cap that glisten like mica.

The cap is initially 1–2.5 cm (1⁄2–1 in) in diameter, oval to cylindrical, but expands to become campanulate (bell-shaped), sometimes with an umbo (a central nipple-like protrusion); finally it flattens somewhat, becoming convex. When expanded, the cap diameter reaches 0.8–5 cm (1⁄4–2 in) with the margin torn into rays and turned upwards slightly. The color is yellow-brown or tan often with a darker center, then pale yellow or buff from the margin inwards. The cap margin is prominently grooved almost all the way to the center; the grooves mark the positions of the longer gills on the underside of the cap. When young, the cap surface is covered with white or whitish shiny particles, remnants of the universal veil that covers immature specimens. The particles are loosely attached and easily washed away, so that older specimens are often smooth. Coprinellus micaceus is hygrophanous, meaning it assumes different colors depending on its state of hydration.