The structural and chemical features of two obsidians from the pre-Hispanic quarry in the town of Nopalillo, Sierra de Pachuca, Hidalgo, Mexico are presented here. The obsidians were collected at a single site. The most common sample is green, glassy and transparent and the other is green-greyish and opaque. Both samples were found to have similar elemental composition.In order to explain the differences in colour, we studied the morphology and structure of both samples using various techniques. Mössbauer and electron paramagnetic resonance (EPR) techniques showed that both samples had the same iron oxidation states as the parameters and relative intensities (%) of the signals from Fe 2+ and Fe 3+ were similar, although in the green-greyish obsidian a low-spin Fe 3+ species was found which was not observed in the green obsidian. X-ray diffraction (radial distribution function) showed that the short-range order due to interatomic distances was the same for both samples; however they differed in the long-range order. Finally, electron microscopy revealed that the two materials differed clearly in their macroscopic structure. The green obsidian surface was found to be smooth and homogeneous and the green-greyish obsidian surface full of large crater-like holes. The presence of holes did not affect the surface area of the green-greyish obsidian.