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Geology/What kind of rocks are these


Have like 6 more this size but different colors and size
Have like 6 more this  

Large one
Large one  
We dug these up while starting a garden in Austin mn. They are very heavy for their size the large one weighs at least 75lbs. Are the volcanic rocks?

Hi Kerry,
You may be correct that the original rock was volcanic, but what is left appears to be mostly agate.  Both volcanic rock and agate are common in Minnesota.  We don't fully understand how agate forms but it is pretty certain that a silica rich solution fills void spaces in rock.  Given time the solution solidifies into agate.  Volcanic rocks from the Superior Region are noted for their vugs (large voids caused by gas pockets#.  Those voids are often filled with minerals.  

Your specimen looks to have been mostly interconnected voids.  They were filled with agatized mineralization.  White colored minerals line the spaces and fill some of them.  Here's a picture of how chaotic an agate can be;

This specimen is solid but yours is not.  All of the empty space was once filled with the parent rock, perhaps basalt #a type of volcanic rock common to the area). Agate is a much more durable mineral than basalt, so as the basalt eroded away over the years what remains is the interconnected lacework of the agate.

Very nice!!

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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