Geode with agate and quartz. (cut & polished surface; field of view: ~3.8 cm across)

Shown above is the interior of a geode. Geodes are small to large, subspherical to irregularly-shaped, crystal-lined cavities in rocks. They form when water enters a void in a host rock and precipitates crystals. The most common geode-lining mineral is quartz.

This geode is lined with thin layers of bluish-gray material called agate. "Agate" is a rockhound/collector term for irregularly concentric layers of microcrystalline, fibrous quartz (chalcedony - SiO2). Agate is quartz. The glassy gray material that covers the agate is macrocrystalline quartz (= top part of photo).

Geode info. from the Field Museum of Natural History (Chicago, Illinois, USA):

"Geodes are hollow, subspherical bodies, ranging from an inch or two to a foot or more in diameter. Most geodes occur in limestones, rarely in shales. They have an outer chalcedonic silica layer which is separated from the enclosing limestone matrix by a thin clay film. The inner surface of the chalcedonic layer is usually lined with inward projecting quartz crystals, though in many geodes drusy coatings of calcite and dolomite occur commonly. Of less common occurrence, are crystals of magnetite, pyrite, sphalerite, and a few other such minor and rarer constituents.

The mode of origin of geodes in sedimentary rocks is but imperfectly understood. That geodes originate in an initial cavity, such as the unfilled space within a fossil, is well recognized, but whether such a cavity is a necessary prerequisite is open to question; geodes may originate in cavities formed by solution.

Many geodes show evidence of expansion, apparently resulting from pressure. A notable example of this singular phenomenon of expansion of the growing geodes is the "exploding bomb" structure. "

Photo gallery of quartz: