Calcite, calcuium carbonate, CaCO3. Creigau quarry, near Cardiff, south Wales. A collection of twinned scalenahedral crystals of calcite interspersed with much smaller nail head crystals. All the crystals have acquired a coating of a reddy brown colour, probably with an iron content. Close to where this specimen was collected was a five foot cavity in the quarry wall, completely lined with even larger crystals, but impossible to recover without catastrophic damage.