MINERAL OF THE MONTH: August and September 2010 – Polyhedroid Agate

The mineral of the month for August/September is the Polyhedroid Agate. The unique polyhedroids are strange looking agates with geometric shapes that have smooth flat sides. Their shape appears more angular than the typical almond-shaped agate, but there is no consistency to the angles of formation. Most are either triangular or trapezoid and hollow, with the bands running parallel to their shape. Although no one knows for sure how these agates formed, there are at least two theories. Some believe that polyhedroid agates are pseudomorph replacements of other, more angular crystals. Another hypothesis is that they formed in spaces between other crystals, perhaps calcite. Unlike the rest of the Brazilian agates found in the southern part of the country, these unusual agates are found at a single site in the northern state of Paraiba. There has been no mining of polyhedroids in recent years. Most were mined during the 1970s. Believe it or not, a few polyhedroids have also been found in the Lake Superior region and in Wyoming.


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