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Geology/Green Obsidian in Crystal Lake, Benzie County Mi


I found a good bucket full of green obsidian in crystal lake. There is more than what I could carry in a bucket an I didn't want or need that much, but there is a plethora. I am just curious as to how it may have got there. It's raw and unfinished and it seems like it may be part of the natural geology of the lake.

Hi Angela,
It is highly unlikely that you have natural obsidian. There was no volcanic activity in the LP of Michigan so obsidian could not be deposited here.

However, there was plenty of volcanic activity in the western UP, and obsidian has been found there. Recent glaciation (about 10,000 years ago) that started in Canada and reached as far south as parts of Ohio and Indiana did scrap off a lot of rock from the UP and deposited in the Lower. Lots of mineral crystals, even diamond, have been found down here so it is possible that a cache of obsidian found its way to Crystal Lake.

But, as I said, highly unlikely. It is MUCH more probable that you have pulled out some glass slag. There is a LOT of that around. For over a hundred years glass slag has been used for many purposes including, concrete additive, fill, railroad ballast, acid mine drainage mitigation, soil additive and jewelry.  

To give you a little background; Slag is the solid material resulting from the interaction of flux and impurities in the smelting and refining of metals. The solid product generally forms a silicate glass-like material, which is primarily nonmetallic. Glass slag can be found in purple, brown, blue and green colors. It can also be banded in different colors, hence its use in jewelry.  

Since it has so many uses slag was/is transported all over the map.  Most of it was used but some got stockpiled and others just dropped off trucks or was misplaced. There's no saying how your cache came to be where it is. But it is a nice find and I hope you are able to make good, decorative use of it.

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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