Amethyst
(c) 2002 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Amethyst is the purple variety of the mineral quartz. Amethyst owes its color to the presence of iron atoms in the crystal matrix, which otherwise consists of silicon and oxygen in the proportion SiO2.
This amethyst slab is a piece of a geode, and the image shows a pavement of amethyst crystals seen end-on. The image below is an edge view of the slab, showing the points of the amethyst crystals. For a better idea of the full crystal shape, see the Herkimer diamond page. And see more examples of quartz in the Quartz Picture Gallery.
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