At Boudi, quartz is found either scattered in a silt-sandstone matrix or within cavities of a complex fault and fractures system. Small openings are completely filled with interlocking quartz crystals. In the reddish mudstones, floating quartz specimens are always found as single crystals without matrix, usually singly terminated but sometimes doubly. The amethyst clearly formed by hydrothermal deposition, but the condition of the crystals suggests they were removed from the original site of crystallization and subsequently encapsulated in the mudstones. The deposit could be regarded, then, as both primary and secondary. The authors hypothesize that the well-shaped quartz crystals originated in the faults and veins in the surrounding limestone, and then eroded and redeposited in the residual mudstones.