The development of the laser technology for stone cleaning purposes has been undermined in the recent years because the technique occasionally conveys a yellow hue to the cleaned surfaces. Several components of black gypsum crusts have been suggested to be potentially responsible for the phenomenon, more specifically some granular fractions of their components. The contribution of these particles to laser yellowing is studied here on powdery deposits collected at Saint-Denis Basilica (France). The powder is characterized by optical and scanning electron microscopy, X-ray diffraction and Raman spectroscopy before and after the laser irradiation. Irradiation of the samples results in a significant yellowing, along with particle fragmentation, while no mineralogical change is evidenced by powder diffrac-tometry. Hematite and goethite micro-Raman signatures ob-V. Verg es-Belmin Centre de recherche en conservation : Laboratoire de recherche des monuments historiques, (CRC:LRMH), CNRS/USR 3224-Sorbonne Universités-29, rue de Paris-77420 Champs sur Marne-FRANCE Tel.: +33 1 60 37 77 80/95 Fax: + 33 1 60 37 77 99 tained before laser irradiation are replaced after irradiation by a strong fluorescence in addition to weak goethite signatures in Raman spectra. The powdery deposit is further analyzed by diffuse reflectance spectrometry after a specific preparation which increases both its cohesion and its content in new phases potentially formed. The powdery material shows a phase with a spectral signature similar to goethite which subsides after laser irradiation. The yellow appearance caused by laser irradiation seems to be due, in part, to a selective elimination of the black components (soots) from the powdery deposit. This elimination reveals a colour which could be related to a pre-existing phase containing iron, possibly goethite or another phase containing iron and having the same spectral signature in the visible range.