Here is a superb milky quartz pseudomorph of calcite that was collected in the legendary Crystal Cave, Ouray District, Ouray County, Colorado. The top surface features sparkling, highly lustrous pure white milky quartz crystals, and the back side shows the original calcite crystals which were grown over by the milky quartz.  A hole in one of the crystals shows that there is a small space between the calcite and the quartz.