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Question
ROCK
ROCK  
CAN YOU HELP IDENTIFY THIS ROCK STRUCTURE IN THE PICTURE.

IT APPEARS TO BE A ROUND CANNONBALL SHAPED STONE THAT WAS ENCASED WITHIN ANOTHER TYPE ROCK. I AM TOLD IT WAS FOUND IN A FARM IN MINNESOTA USA.
ALSO THAT THERE WAS A TOP COVER AS WELL AS THE BOTTOM THAT IS NOW LOST. BUT WHEN YOU REMOVE THE BALL FROM THE SHELL YOU CAN TELL THAT THE TWO WERE SOMEHOW COMPILED TOGETHER AS THE GRAIN LINES ON THE BALL MATCH THE SHELL HALF.
I DONT KNOW IF IT IS A METEOR OR NOT BUT IT DOES SEEM SLIGHTLY HEAVER THAN MOST ROCKS
PLEASE HELP IF YOU CAN  

THANK YOU WAYNE

Answer
Hi Wayne,
You have a concretion.  Concretions are hard "bodies" that form in sediments. As they lithify into sedimentary rocks chemical changes that may be due to microbial activity, cause minerals to precipitate out of the surrounding groundwater and cement the sediment together. Most often the cementing mineral is calcite, but sometimes iron-bearing minerals can do the job.  

Concretions can take the shape of anything from cylinders, to blobs, to perfect spheres. They can be as small as a pea or as large as a house.

It isn't certain what causes concretions.  Often there's a particle in the center, such as a fossil. Others don't have anything inside, but the prevailing thought is that something caused the minerals in the groundwater to precipitate out around it.  

Here's a few pics of a concretion that looks much like yours.  

http://formontana.net/concretion1.jpg
http://formontana.net/concretion2.jpg
http://formontana.net/concretion3.jpg

It appears your sample has more iron in it's formation than the examples I sent because of it's darker color and extra weight.

Hope this helps.
Bob

Geology

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C. Robert Reszka, Jr.

Expertise

I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.

Experience

I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Organizations
Michigan Basin Geological Society

Publications
Decade of North American Geology.
Bedrock Geology of Michigan

Education/Credentials
BS Wayne State University

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