A hopper crystal is a form of crystal, defined by its ‘hoppered’ shape: the edges are fully developed, but the interior spaces are not filled in due to rapid growth. This results in what appears to be a hollowed out step lattice formation; the interior edges 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.
Hoppering occurs when electrical attraction is higher along the edges of the crystal, causing faster growth at edges than near face centers. This attraction draws the mineral molecules more strongly than the interior sections of the crystal, thus the edges develop more quickly. Hoppering is common in many minerals, including lab-grown bismuth, galena, quartz (called skeletal or fenster crystals), gold, calcite, halite (salt), and water (ice).
The Daily Omnivore 
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