Hopper crystal

  A hopper crystal is a form of crystal, defined by its "hoppered" shape.

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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.

Hopper crystals get their form from the fact that they tend to grow more rapidly at the edges of each face than at the center due to the higher electrical attraction present along the edges of the crystal. 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).