Some botryoidal goins' on!

Agate is a semiprecious gemstone that is a variety of chalcedony, a type of quartz:

Appearance
Agate is known for its concentric bands of color, which can be white, gray, yellowish-brown, reddish-brown, blue, or green. The colors come from iron oxide minerals in the quartz.

Formation
Agate forms in cavities in volcanic and metamorphic rocks over thousands of years. Silica-rich water deposits quartz crystals in layers that are parallel to the cavity walls.
Uses
Agate is a popular choice for jewelry and carvings because of its beauty and toughness. It has been used to make beads, cabochons, and cameos.
Origin
The name agate comes from the river Achates (now Dirillo) in southwest Sicily, where it was discovered. However, some modern etymologists believe the river was named after the stone.
Other names
Agate has many names, including eye agate, lace agate, and moss agate.
Location
Agate is found all over the world, but Brazil is the main commercial source. In the United States, the western states of Oregon, Washington, Idaho, and Montana are the main sources.