Complete crystal of Polyhedroid Agate. This geometrical Agate is unique in the world: discovered only one time in the world in late 1970's, in the Brazilian state of Paraiba. Agates are never geometrical, but... Some suggested that the polyhedroids formed in the interstices between tabular crystals of calcite. And other are thinking of a pseudomorph after feldspar (replacement of the feldspar crystal by agate due to dissolution) or… we don’t know for sure! It happened once, and might never be seen again, as the area is now under the water of a dam.