Agates are formed in volcanic lava which contained gas bubbles that filled with water and turned to quartz and sometimes amethyst.
Agate is known for its bands of color formed by intermittent deposits of quartz into irregular cavities in volcanic rocks. The concentric wavy patterns and colors are caused by impurities. Agate is a banded, multicolored, variety of Chalcedony. It is translucent meaning you can see through it.