Naica´s ”Cueva de los Cristales” was discovered in 2000, and since then it has been considered particularly interesting for its beauty and for the challenges it poses to crystallography. This article focuses on the study of the wall-selenite interface by various techniques, particularly X-ray diffraction (XRD), scanning electron microscopy (SEM), with emphasis on micro-X-ray fluorescence (-XRF) and micro-X-ray absorption near edge structure (-XANES). The main phases calcite, quartz, goethite and montmorillonite were identified by XRD, as well as the association of crystalline and amorphous minor and trace phases of Zn, Mn, Cu, As and Pb. The later were identified in -XRF maps and -XANES spectra. The results for the morphology and the chemical description of the crystal-wall interface may contribute to propose a nucleation and growth mechanism for Naica megacrystals.