2.75" dia. polished agate sphere. Agates are semi-precious gemstones that are a variegated form of chalcedony (a translucent variety of quartz of various colors and waxy luster). Agates naturally develop when an empty pocket inside a host rock is filled molecule-by-molecule, layer-by-layer as microcrystals form concentric bands or other patterns. The colors and arrangement of the microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process. Unlike other gemstones, each agate is unique. Even slabs cut from the same specimen will vary in color and design. Varying shades of gray and white, each layer is visible on the outside while the sphere has been polished. This example was found in Rio Grande do Sul, the southernmost state in Brazil. Comes w/a wooden display base. Exc.