Kansas Pop Rocks, also known as Boji Stones, are mined in Smoky Hill Chalk Member of the Niobrara Formation within Gove County, Kansas. 

Some say Pop Rocks began as fossils from the Cretaceous period, around 80 million years ago. Over the years the fossil material was replaced by Pyrite, Iron Magnetite, Marcasite, Jarosite and/or other minerals formed around the fossilized core which you can often see the original embedded fossil. However, others say this is not true, that metamorphic rocks are completely absent. Instead, "these iron sulfide concretions were created by the precipitation of iron sulfides within anoxic marine calcareous ooze after it had accumulated and before it had lithified into chalk".  

They received their name after being thrown into a fire which it exploded in a pyrotechnic display (please don't try this at home). They produce sparks and a burning sulfur smell when cut or hammered. They are said to be female and male stones with the females being more smooth and oval and the males are rougher with some protrusions. 
 
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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.