Agate can be an exceptionally colourful stone that's often but not always characterized by concentric bands. It has a long history that can be traced back thousands of years.
During the Middle Ages people wore agate in the belief it would cure insomnia and bring sweet dreams.
Agate is one of the best known varieties of the mineral chalcedony. It's relatively tough with a fibrous structure. Layers can be translucent to opaque.
When free from impurities colours tend to occur in shades of white and grey. With the presence of other minerals it can be almost any colour at all. The colours and banding that can be seen in most agates are caused by impurities of other minerals.