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Naming Geodes
Geodes are given a variety of names. The word “geode” is often preceded by the name of the mineral material which has filled the geode. “Agate geode” and “amethyst geode” are examples. The word “geode” might also be preceded by a geographic or stratigraphic name. “Keokuk geode” and “Brazilian geode” are examples.
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Sedimentary Geodes
Geodes in sedimentary rocks are usually found in limestones, dolomites, and calcareous shale. In these deposits a gas-filled void can serve as the opening for geode formation. Shells, tree branches, roots and other organic materials often decay away to leave a void for the formation of mineral materials. These cavities can be filled with quartz, opal, agate or…
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Volcanic Geodes
The most widely known and sought-after geodes are those that formed in areas of volcanic activity. Voids in basaltic lava flows often are infilled with agate, quartz, opal and other material delivered by hydrothermal water or groundwater. Some voids are spaces occupied by gases that failed to escape the lava flow before its surface crusted over. Where…
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Geologic Occurrence and Formation
Geodes are not found randomly here and there. Instead they are usually found in large numbers in areas where the rocks have formed in a special geochemical environment. Most geodes localities are in A) stratified volcanic deposits such as basalts and tuffs; or B) stratified sedimentary carbonate deposits such as limestones and dolomites. A diversity of other environments yield a small…
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The Public’s Love Affair With Geodes
Most geologists enjoy geodes. However, the general public has a love affair with them. They are delighted and amazed that an uninteresting rock can contain a beautiful cluster of gemmy crystals, or a colorful lining of banded agate, or both of those in the same cavity. Broken open, or sawn and polished, people who have…
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What Are Geodes?
Geodes are spherical to subspherical rock structures with an internal cavity lined with mineral materials. They have a durable outer wall that is more resistant to weathering than the surrounding bedrock. This allows the geode to survive intact when the surrounding bedrock weathers away. The mineral lining the cavity is often a scintillating druse of tiny quartz crystals underlain by multiple…
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A “Gemmy” Architectural Stone
Some deposits of anorthosite are quarried and cut into slabs that are used to manufacture small sculptures, countertops, window sills, tiles, facing stone, and other architectural products.
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Notable Labradorite Localities
Labradorite is named after its location of discovery on the Isle of Paul, near Nain, Labrador, Canada. It was discovered there in 1770 by a Moravian missionary. Labradorite with superb labradorescence is produced from a few deposits in Finland. The best of this material was given the name “spectrolite” by the director of the Geological…
