An impressive agate in the nodule in which it formed. The face of this agate has been polished and reflects light beautifully. The outside of the nodule has been polished just enough to achieve a smooth finish.
Agate is a type of microcrystalline quartz that occurs in a nodule that's encased within a host rock. The nodule is hard and resilient so usually survives long after the rock has eroded away. Agate mostly forms in rocks which are volcanic.
The colour and patterns in agate are caused primarily by impurities of other minerals. As the layers or bands within the nodule build up, they follow the shape of the cavity. Once full the minerals then crystallise into one solid mass. Should the void not fill completely, macrocrystalline (larger and visible with the naked eye) quartz crystals will grow.