What is a phantom in mineralogy ?

Phantom : definition

A "phantom" crystal is a crystal which has impurities organized on internal faces, materializing one or more stages of growth. In quartz, these impurities usually form nested pyramids and provide informations on the crystallization conditions of quartz. Impurities are often clays or fine crystals of hematite or chlorite, which settle on the faces of a crystal being formed.

Although quartz is the best candidate for showing these growth figures due to its clarity, any crystal that is sufficiently transparent can exhibit them.