Fossil Shell is mostly Dolomite, Marble and Calcite with swirls of limestone, chert, and other impurities.  It has beautiful gray, white and transparent markings making it appear to be a fossil. 
 
It's also known as Marbled Shell Jasper, Sea Shell Jasper, Shell Marble, Shell Obsidian, even though it's not a Jasper nor an Obsidian.
 
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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.