GHOST QUARTZ

Either one or more or, rarely, all of the needle phases, eaten away as if by a solvent, are marked with a multitude of tiny, sparkling caverns. The crystal has not lost any of its transparency or shine. Its ability to reflect and refract light is multiplied by the increased volume of the cavities. Each wall retains enough smooth and pristine surface to remind the viewer that it was once perfect, without any roughness.

(R. CAILLOIS, Stones, p. 80)