Nearly all rocks are composed of minerals. Fine specimens are rare and ten to occur in fissures and cavities where the crystals have been unobstructed during their growth.
Agate is a form of chalcedony, These silica based stones form concentric or irregular layers, usually lining a cavity. The bands are usually variegated in shades of white, grey, green, blue and black.
These swirly agates reminded me of the illustrations on weather patterns in this book about preflight aeronautics