A small cluster of eisenkiesel crystal, which originated from fossil stromatolites built from and including threads of bacteria or fungi mycelia. The Hohenlimburg quarry is the same geological setting as Warstein. As far as I know up to date the only localities worldwide for this material are in the Sauerland area - "type locality" Warstein. Here's an extract from a German publication: "Fossil fungi in iron stromatolites from Warstein (Rhenish Massif, Northwestern Germany) Zeitschrift Facies Verlag Springer Berlin / Heidelberg ISSN 0172-9179 (Print) 1612-4820 (Online) Heft Volume 7, Number 1 / Dezember 1982 DOI 10.1007/BF02537228 Seiten 237-259 Fossil iron stromatolites, formed by fungi, have been found near Warstein, W. Germany. The stromatolitic and massive ferruginous quartzes are always confined to karstic hollow molds of the Devonian "Massenkalk". The few occurrences show that these are chalcedony-quartz-silicifations of mainly ac-oriented, corrosively widened cracks. A mineralization temperature of about 120°C was found by microthermometrical measurements. Druses in ferruginous quartz sometimes carry clear, prismatic to pseudocubic quartzes embedding fungal mycelia. The fungal filaments are generally encrusted with goethite and appear in stromatolitic, massive and patch-like forms. The fungi can be referred to the order Mucorales and the genera Chaetomium, Preussia, and Papulaspora. An Oligocene/Miocene age is presumed ... " Newer texts suggest that the fossil inclusions are threads of bacteria archaea - but I haven't seen these publications so far.