There are two main groups of quartz: chert and chalcedony. Chalcedony is composed of fibrous crystals intergrown, yielding a variety of patterns including agate. Agate is a variety of chalcedony composed of concentric bands of colors. Agates form in cavities left by gas bubbles in cooling lava. Silica-rich groundwater builds layers of crystals on the cavity walls.
Image Number: MSA 248