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Dwarves' Earth Treasures Online Rock Shop:
Polyhedroid Agates
Pariba, Brazil, South America
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.    Those agates here may be considered strangest of all agates because of their strong geometric shapes, hence, "polyhedroid agate". They were first found in the clay deposits formed from decomposition of rock matrix, but the source turned out to be short-lived (don't know if that's true or not). No one was sure of how those agates were formed since their shapes did not match any crystal structures of various minerals that the agate may had replaced. I have found several evidences/clues (pictures at the bottom of this page) that those polyhedroid agates may be a form of agate-filled gaps between the large calcite/selenite blades in the cavities (more evidences can be found in Johann Zenz's Agates book). I found and cut a large thunderegg at Rockhound State Park, Deming, New Mexico, and its cavity is completely filled up with black bladelike calcite crystals with agate filling in the gaps. The shapes of the agate filled gaps are strongly geometric (triangles, trapezoids, and other polyhedroid shapes).  Most polyhedroid nodules are quartz geodes. Black, gray, white seem to be most common colors of the polyhedroid agates.

Updated October 2010. new specimens have 2010 in their item names
All Slabs have both sides polished unless stated otherwise.

2010yPOLYSLABn007-SOLD POLYSLAB2008n24-$60. almost perfect right triangle shape
only one side polished a fracture
POLYSLAB2008n25-$80