These crystals are called 'frosty diamonds' if you grow them from the Smithsonian crystal geodes kit. You can get similar results if you grow the crystals yourself using alum and a small rock in a bowl.
Alum Crystals
From Anne Marie Helmenstine, Ph.D., About.com
In the Smithsonian kits, these are called 'frosty diamonds'.
Anne HelmenstineGrow Your Own Crystals
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