Septarian nodules were formed on the bottom of the ancient Gulf of Mexico sea floor and have also been found off the Madagascar coastal waters. At the time, volcanic activity disturbed decaying sea life, making them roll around, collecting mud. The water trapped inside the mud ball concretion created the yellow calcite crystals.
 
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* Sizes, shapes and colors my vary due to being a natural stone, being hand-carved and/or difference in computer monitors.