CAP 4–15 cm across, obtusely conical when young, expanding to convex or nearly plane with broad umbo. Margin inrolled when young, becoming wavy and uplifted; outer edge inrolled at all stages. Surface smooth, viscid to glutinous when wet, tacky when dry. Color variable, often variegated with various shades of smoky violet, pinkish, lilac to purplish brown, typically with a darker disc, and dark spots or splotches. As it ages, brownish and orangish tones become more prominent; dark vinaceous brown, dark reddish brown or dark brown when old. Dry caps metallic, and darker: blackish brown to vinaceous, or reddish brown. GILLS narrowly attached to appearing free, crowded, broad. Whitish to pale pink or grayish when very young, soon developing smoky tones, and dark yellowish gray in age. STIPE 3.5–10 cm long above ground, up to 25 cm long overall, 0.7–3 cm thick at apex, equal or swollen towards ground level, tapering to an often deeply rooted pseudorhiza underground. Surface dry, smooth to appressed-fibril- lose, longitudinally lined (occasionally the cartilaginous rind will crack longitu- dinally). Pale pinkish gray to purple-gray at apex, transitioning to a deep purple- brown or red-brown lower portion when young, darkening from the base up in age. Pseudorhiza deep red brown overall. FLESH firm, thin, except at disc; whitish, tinged with pink or purplish tones in cap, slowly staining cinnamon. Stipe solid, with a dark cartilaginous rind, and firm pallid to whitish pith, slowly staining cinnamon brown when cut. ODOR slightly floral, or sweet farinaceous. TASTE farinaceous to slightly bitter. SPORE DEPOSIT pale purplish to dull yellowish brown. MICROSCOPY: Spores 8.2–10 x 5–6 μm, averaging 9 × 5.5 μm, fusoid-el- lipsoid in face view, somewhat lemon-shaped in side view, coarsely roughened, except near beaked apex, with eccentric apiculus. Amber in KOH, dextrinoid in Melzer’s reagent. Basidia 27–38 x 6–7.5 μm, clavate, with a short pedicel, 4-spored. Cheilocystidia forming a dense gelatinous layer on gill edge. Highly variable in length and shape; a mix of narrowly and broadly clavate elements, with tips 3 μm when young, to 5–7 μm when mature. Often with long filamentous outgrowths when old. Pleurocystidia absent. Cap cuticle a two-layered ixocutis; upper layer 100–300 μm thick, consisting of radially aligned, cylindric, highly gelatinized, hyaline hyphae, 3–6 μm across in a tightly compacted gelatinous matrix. Lower layer 50–100 μm thick, dull orange brown, made up of slightly thick-walled hyphae, 5–8 μm across. Clamp connections absent.
ECOLOGY: Ectomycorrhizal with conifers (and possibly Tanoak, Notholithocarpus densiflorus), apparently restricted to mature and old-growth forests. Fruit bodies solitary or scattered in small patches, appearing in fall and early winter.
SIMILAR SPECIES: Fruit bodies of Phaeocollybia redheadii are typically slightly larger, have reddish brown to dark brown caps (even when young), long rooting stipes stuffed with cream-colored pith, and larger spores. Older specimens of P. benzokauffmanii can be difficult to distinguish macroscopically; in such cases, P. redheadii can be distinguished by its larger spores measuring (8) 9–13 x 5–7.3 μm, averaging 10.5 × 6 μm. Phaeocollybia spadicea has a dark brown to gray-brown cap that develops vinaceous tones in age; it is typically smaller (caps 2.5–12 cm across), has darker gills, slightly smaller spores (8 × 5 μm), and has tibiiform to lageniform cheilocystidia with narrow, thick-walled necks. Phaeocollybia oregonensis has a grayish to grayish brown cap, smoky-white to dark yellowish brown gills, a solid stipe stuffed with white pith, with a grayish pink apex and dull yellowish cinnamon spores. Microscopically, it has small, nearly smooth spores, (6.8 × 4 μm), and thin- walled, narrowly clavate cheilocystidia, which develop apical outgrowths in age. Phaeocollybia kauffmanii has a viscid, orange-brown to warm brown cap, a solid stipe with a pinkish buff to cinnamon buff apex, and a strongly farinaceous odor and taste.