Geodes are hollow, closed shells made of igneous or sedimentary rock. They generally form in cooling lava or in sedimentary geological structures over several thousand years. Though the mechanisms of void formation differ, the same process occurs in time for all geodes: water-carrying minerals, usually calcite (CaCO3), seeps into the microscopic pores of the shell and accumulate the inner lining that, in the final product, is a visually pleasing crystalline structure,.
Since these rocks are closed surfaces, the only practical ways to view the inner crystals are to break them or to use a form of imaging that makes use of light outside of the visible range like x-radiography. A two-dimensional x-ray image will be able to detect that the rock is hollow, but it will not necessarily include information about the varying thickness of the shell or the shape of the crystal lining. A Digitome volumetric exam, however, is able to detect the contour of the void within the rock in a non-destructive manner.
The elemental make-up and thinness of the geode shown in the video allow for easy penetration of x-rays that are relatively low in energy. The quartz is mainly silicon dioxide and the crystal lining’s most absorptive element is calcium, with an atomic number of 20. The exam in the video was taken with an x-ray beam of 80 kVp; metallic elements like iron, of atomic number 26, often require more energy for adequate penetration. Since calcium composes much of the geode, this material is a good model for concretions that would be found on the ocean floor in underwater archaeology excavations. In particular, the geode shown in the video was volumetrically examined to prepare for taking three-dimensional x-radiography exams of artifacts in concretions from the Queen Anne’s Revenge.
Scanning upward through the geode, it is evident that much of the rock is indeed hollow. Areas of particularly low or high thicknesses are identifiable through the exam and can be accurately located on the actual geode. Because of this, one could carefully break open the geode or open a small hole by accessing an area of weakness found in the 3D exam. This potentially provides the opportunity for a cleaner cut of the rock for museum display, personal use, or commercial purposes.