Systematic mineralogical investigations, carried out on panned stream concentrates from the northwestern part of the Cambro-Ordovician Stavelot massif (Belgium), led to the discovery of nodular gray monazites. These were later identified in situ in black slates associated with conglomeratic levels containing phosphatic material. Microscopic investigation shows that gray monazites are authigenic and predate cleavage. To test the hypothesis of a sedimentary origin for rare earth elements, samples of shales, phosphatic conglomerates, and monazite concentrates were submitted to sequential inductively coupled plasma analysis. Results show that the rare earth element pattern in gray monazites is a reflection of the abundance of these elements in the host rock. This suggests a model invoking desorption of the rare earth elements from their clayey support and differential precipitation of rare earth phosphates in the sediment initiated by P mobility during diagenesis. The genetic model for gray monazite mineralization assumes three essential and complementary phenomena: (1) sedimentation of clays in a reducing marine environment, (2) a phosphate anomaly in the same lithological sequence, and (3) evolution to low-grade epizonal metamorphic conditions.These nonrestrictive genetic conditions clearly suggest that gray monazites have been generally overlooked and must be much more widespread than believed. The economic interest of gray monazite lies in potential stream or beach placers. Their low Th content and comparatively high Eu content are interesting features in the scope of ore beneficiation.