In this work, siliceous breccia, a natural rock powder composed essentially of SiO2 α-quartz, has been employed directly as a catalyst without any chemical treatment for the synthesis of carbon nanotubes (CNTs) via chemical vapor deposition (CVD). In addition to quartz, it contains dispersed micro-inclusions of iron oxide-hydroxides, goethite or hematite, which act as catalysts to dissociate the hydrocarbon precursors and form carbon nanostructures. The catalytic performance of this powder was evaluated for C2H4 decomposition at 750 °C, with and without H2 flux. Thermal oxidation stability and carbon yield were measured by means of thermogravimetric analysis. Structural and vibrational characterization of the resulting material was carried out by environmental scanning electron microscopy (ESEM) and micro Raman spectroscopy. ESEM images show that the H2 addition affects the CNT diameter. We …