File:Geode (Soledade, Rio Grande do Sul, Brazil) 2 (32233840960).jpg

Shown above is the interior of a moderately large 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 interior is coated with various forms of quartz. The larger, blocky-shaped masses were formerly dogtooth spar calcite crystals, but they have been replaced by quartz. The smaller, rounded bumps are chalcedony (= fibrous, opaque to translucent, microcrystalline quartz). Everything is coated with finely-crystalline, drusy quartz.

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. "

Locality: unrecorded/undisclosed site at or near the town of Soledade, Rio Grande do Sul State, far-southern Brazil

Photo gallery of quartz: