This 55 mm quartz crystal from Diamantina, Brazil shows a terminal flat face (pinacoid), about 2 mm across, perpendicular to the prism faces. Seen close-up this face is a carpet of tiny dissolution pits. The rhombohedral faces show tiny downward-pointing triangular dissolution pits. The prisms show upward-pointing triangular markings.
Anotherr specimen displaying dissolution c faces is the Arizona amethyst.
I just love your minerals and fossil specimens. Great photos. I have collected some very neat quartz crystal crystals with the features shown in # 14 and 15 photos I have collected here in Georgia west of Atlanta. Some of the crystals will be deformed but the surface features will reflect light on the same plane. Very Fascinating. Thank you for sharing !