A geode is a hollow rock, sometimes with a fantastic growth of glittering crystals within it and sometimes lined with silky forms of chalcedony such as striped agate, jasper, or opal. Some geodes hold stunning combinations of smooth stone and sharp crystals. Because they’re unpredictable from the rough outside, learning the contents is the delight of opening a geode.
A geode begins as an empty space. One may form where a root or bit of debris rotted away or where an animal burrowed. Layers of rock may slowly cover a hollow in sedimentary rock, or hardening lava may quickly cover a gas bubble in igneous rock. Either way, geologists call a geode’s shape subspherical, meaning it ends up rounded, but is not a perfect sphere.
The process of geode formation takes centuries. Rocks for kids.com estimates that it takes 240 million years to form a geode. Usually, groundwater soaks through rock above the roughly spherical void, dissolving out minerals and depositing them within the nascent geode. Mineral-rich hydrothermal water may also flow upward from geysers or hot springs.
If deposited minerals grow relatively quickly and crowd together, they form microcrystals so small they will appear as smooth bands of color. In other situations crystals have room and time to grow, and can reach full form. Such geodes may be filled with black quartz, rock crystal, or sparkling spears of amethyst.
Minerals penetrate some geodes for so long that they fill up entirely; they have become nodules. Some geodes, called rattlestones, have loose pieces inside.
Thundereggs, lithophysae, are geode relatives. According to the USGS, the United States Geological Survey, thundereggs weren’t ejected from volcanoes as volcanic bombs, they aggregated in beds of soft volcanic ash. Dissolved silica permeated masses of “cinders,” by which geologists mean fragments of volcanic rock, not the remains of a fire. Eventually, chalcedony began to form. However, before the chalcedony could fully solidify, the center of the accretion split apart, perhaps because of irregular shrinking. This allowed other materials to enter the thunderegg. Therefore, thundereggs often have breathtaking star-shaped centers full of agate, jasper, or opal.
Ironstones are nodules, while ironstone is a rock. According to Concretions and Nodules in North Dakota, written by Bob Biek for the North Dakota Geological Survey, what most people call ironstones formed at the same time as the sediments around them, probably under shallow water, and possibly as a gel. Ironstones with alligator skin surfaces may have been caused by the stone’s surface drying and cracking, at which point the still moist interior material flowed out to form low ridges along the edges of the polygonal plates of the rock surface.
Though they are mysterious and beautiful, ironstones and thundereggs are not true geodes, in spite of their shape and appearance. However, a cave on an island in Ohio is a geode, though people can stand inside it.
On South Bass Island, east of Toledo in Lake Erie, anyone can stand inside a geode lined with crystals of blue celestite that are from 8 to 18 inches long. Another geode cave, this one lined with transparent gypsum prisms, was discovered in 2000, in Almeria, Spain. Geodes are hollow rocks, lined with a fascinating variety of rocks or minerals. They are as intriguing as a locked room mystery or an unmapped box of chocolates, and as satisfying to open.