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Unusual Rock
Unusual Rock  

 I found this rock in a box that a relative had kept. She lived in Monterey, Ca. however she also traveled a lot so I have no idea where she may have found it. It appears to be a type of Basalt with unusual columns going thru it. I've seen similar columns on volcanic rocks but they are much larger than this one. The column's have both five & six sides and a flower petal like appearance near the edge of each columns with a more, but not completely,  symmetrical shape in the center of the columns. Also the edges of the columns seem to be sewn together or a zipper like appearance. Its a small rock, the same size as my computer mouse and light weight.
  Any ideas?

Hi Raedel,
What you have there is Hexagonaria percarinata, or more conventionally known as Petoskey Stone.  It's a fossil coral that was alive 350,000,000 years ago.  It's probably from the west coast of Michigan, that's the type locality; Petoskey Michigan.  Here's a couple websites that will tell you some more about your rock.

It's very collectable and used for jewelry and sculpture in many cases.  It is a host rock for oil in Michigan and the center of some of the structures are dark because they are filled with oil.  Sometimes, when you scratch the rock you can smell oil.

Hope this helps.


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C. Robert Reszka, Jr.


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.


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.

Michigan Basin Geological Society

Decade of North American Geology.
Bedrock Geology of Michigan

BS Wayne State University

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