Agate is a form of quartz known for its circular banding. It forms in layers over millions of years as groundwater with mineral impurities drip into empty spaces in volcanic and metamorphic rock. The different colors in the layers are due to different concentrations of minerals that form that layer. If you look at geodes, you’ll notice that the base or outside of the geode is composed of agate.

The photo shows the Agate slice backlit- the slice will appear dark blue when displayed with shadows