Nodules: Generally, nodules have no space in them, (while geodes do and are “hollow”). Most nodules like geodes are spherical but occasional irregular shaped nodules occur. Just like geodes, nodules have a competent external lining that allows them to separate from their host rock. As the host rock weathers and erodes, the nodules stay in- tact, separate from the host and become spherical rocks. Some nodules form in a similar fashion to geodes but the space in the host rock has been completely filled in by precipitated cryptocrystalline or crystalline minerals. Others may be formed by crystallization of minerals during deposition of the host rock. Other nodules may form by mineral replacement of the host. Like geodes, nodules can be layered with a concentric growth pattern. Some can have multiple minerals in them while others may be just one composition like chert (cryptocrystalline quartz) or hematite (iron nodules). In gravel beds many of the agate, chalcedony, jasper and chert pebbles may have started out as nodules in a host rock. They were eroded from that host and then deposited in the gravel. Like geodes, nodules can have a wide variety of mineral composition but nodules of cryptocrystalline quartz (agate, chalcedony, jasper, chert, etc.) or opal are the most common. Some nodules may have an open space with small, often drusy, crystals in it. Is it a geode? Depends on who you ask. If the opening is small, some collectors and rockhounds will still call it a nodule. Others may call it a geode based on the common usage explained here. Who’s right? Depends on the viewer’s point of view and preference.

In the Bighorn Basin chert, agate, chalcedony and other cryptocrystalline quartz nodules are common in the Bighorn Dolomite, Madison Limestone, Amsden Formation, Cloverly Formation, as well as, in Tertiary Conglomerates on the west side of the Basin and in gravel beds along river terraces and riverbeds. Agate and Chalcedony nodules are found in the Absaroka Volcanics.

Vug: Like geodes, vugs are cavities that contain crystals or cryptocrystalline material. The center of the cavity remains open. Unlike geodes, vugs do not have a competent lining so if they separate from their host rock they do not remain intact. Unlike many geodes, vugs probably will not exist after the host rock weathers and erodes away. Like geodes, thundereggs and nodules, vugs are generally spherical.

Miners often call vugs full of large crystals “pockets”. If the minerals have precipitated along cracks, fault breccia, fissures or joints they are generally called fissure or breccia fill. These are long linear crystal veins and are not usually considered vugs. Some rockhounds put a size limit on their usage of vug. Even if it has a competent lining some may call it a vug  if it is small in size. As you can see it can be confusing in a conversation between rockhounds. In the Bighorn Basin vugs are found in igneous and sedimentary rocks. In the Bighorn Mountains and Basin vugs containing calcite are common. The Absaroka Mountains have yielded vugs with various zeolite minerals.


Thundereggs: This term applies to a specific rock structure that occurs in rhyolite and related volcanic rocks that are formed under certain conditions. The voids where the thundereggs forms are called lithophysae. These are spherical structures (spherulites) formed in rhyolitic igneous rocks that have undergone some gas pocket expansion and growth during a highly explosive volcanic event. This rapid gas expansion creates voids, Lithophysae, in the resultant lava flows. After the lava flow is emplaced, low-temperature hydrothermal solutions and/or groundwater become saturated with silica from the original magma and dissolved from surrounding high silica country rock. The water then transports silica in a dissolved state into the voids. The water solution with its high silica concentration gradually turns into a silica gel which eventually hardens into chalcedony, agate, etc., and sometimes opal, to form thundereggs. Sometimes there is space left in the thunderegg and quartz crystals form in the center. In this case you could call it a geode. Because thundereggs form a certain way, they can be a geode, but all geodes are not thundereggs.

Most commonly thundereggs are filled solid with cryptocrystalline quartz and possibly some opal.
Rarely, other minerals could be present. Thundereggs can be from a ½ inch to several feet in diameter. We do not have thundereggs in the Bighorn Basin. Thundereggs have been found in Yellowstone National Park but are not collectable. (Rock collecting is not allowed in National Parks)

Dig Deeper
https://geology.com/articles/geodes/ 

https://www.mindat.org/glossary/geode
Garlick, D. and Jones FT, 1990, Deciphering The Origin of Plume- Textured Geodes: Journal of Geoscience Education, v.38, p. 229 Cocos Geodes, Brazil
https://zh.mindat.org/article.php/1098/Thundereggs 

https://zh.mindat.org/glossary/lithophysae

Greg Jones
March, 2022