<P> A hopper crystal is a form of crystal, defined by its "hoppered" shape . </P> <P> The edges of hoppered crystals are fully developed, but the interior spaces are not filled in . This results in what appears to be a hollowed out step lattice formation, as if someone had removed interior sections of the individual crystals . In fact, the "removed" sections never filled in, because the crystal was growing so rapidly that there was not enough time (or material) to fill in the gaps . The interior edges of a hoppered crystal still show the crystal form characteristic to the specific mineral, and so appear to be a series of smaller and smaller stepped down miniature versions of the original crystal . </P> <P> Hoppering occurs when electrical attraction is higher along the edges of the crystal; this causes faster growth at the edges than near the face centers . This attraction draws the mineral molecules more strongly than the interior sections of the crystal, thus the edges develop more quickly . However, the basic physics of this type of growth is the same as that of dendrites but, because the anisotropy in the solid--liquid inter-facial energy is so large, the dendrite so produced exhibits a faceted morphology . </P> <P> Hoppering is common in many minerals, including lab - grown bismuth, galena, quartz (called skeletal or fenster crystals), gold, calcite, halite (salt), and water (ice). </P>

Why do hopper crystals rarely form in nature