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 and aragonite.
 
Aragonite, like Calcite, forms from calcium carbonate. The difference is Aragonite crystallizes in orthorhombic crystals; whereas Calcite crystallizes in trigonal crystals. Freshwater and marine environments create a biological and physical processes to form Aragonite. Aragonite may also contain sand inclusions, which gives it a brown color.
Aragonite is the main component of many organic substances, such as pearl and coral. The iridescent surface of Pearl and Mother-of-Pearl is actually a layer of Aragonite secreted by mollusks and related invertebrates. [mineralsdotnet]
 
 
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* Photos show all sides.
* Sizes, shapes and colors my vary due to being a natural stone, being hand-carved and/or difference in computer monitors