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May-June 2014

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Mineral Mysteries: Polyhedroids

Polyhedroids, I believe, are generally not known or recognized by the vast majority of the mineral collecting community. What they are is simple to describe; how they formed is a bit more challenging. Polyhedroids are strange closed forms composed of quartz, although I can see no reason why they could not occur as other minerals. They are flat-sided, geometric-appearing blocks of polycrystalline quartz that resemble single crystals, but they exhibit no symmetry. Most of their interiors are at least partly agate; they typically are color banded when cut or broken open, and they tend to be hollow.

John S. White, a consulting editor of Rocks & Minerals, operates Kustos, a museum/collector consulting business. Now retired, he is the former curator-in-charge of the National Mineral and Gem Collection at the Smithsonian Institution.

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