Agates are semi-precious gemstones that are a variegated form of chalcedony pronounced, which is silicon dioxide in the form of microscopic fibrous quartz crystals. Agates naturally develop when an empty pocket inside a host rock fills in molecule-by-molecule, layer-by-layer as these microcrystals self organize to 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.Even slabs cut from the same specimen will vary in color and design.Some agates exhibit a shadow effect in which there is what appears to be movement across the agate layers when the specimen is rocked back and forth. This optical effect only occurs when there are alternating translucent and opaque bands. It is caused when light goes into the agate, but is not reflected out.