Scratch my head...

[Jackmandu]

I found this while out searching for rocks to tumble...very unique with the nodules of ??  and the crevices starting to crystallize.  Anybody want to share their insight on this??

[Myrmica]

I don't know the formations or ages in South Dakota?  I am guessing that is where you found this?  It looks like it could be fossils of brachiopods(?) in which the insides of the molds have formed cavities with some calcite or quartz crystals.  

[grandpa]

Close-ups of the areas inside the broken nodules would be useful in determining their ID.  Also close-ups of the surface of the intact nodules might be useful in determining their make-up.  Finally, if you could get close-ups of the crystals inside the crevices you mentioned, that might add further useful input for an ID.

 

My initial impression is a conglomerate of small geodes with likely chalcedony deposits, possibly quartz crystals, or calcite crystals as part of the conglomerate - an interesting and attractive addition to your collection.

[Jackmandu]

Here are some additional pictures...thanks for all the input and insight!!!

[grandpa]

The crystals in the last photo appear to be calcite.  The interiors in the fist photo appear to be chalcedony.  What I'm seeing confirms for me my initial impression.