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We found this rock near the shore of Sand Lake, in Iosco County, Michigan, across Lake Huron from the "thumb". It is greyish-brown in color. Looks shiny in the light. It has a rough texture. If you rub it with your thumbnail, tiny little bits will flake off. Thanks for your help.

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The rock appears to have two areas of surface texture, a somewhat smooth, rounded area (on the upper part in the picture) and a rough broken area. This would indicate that this was transported by a glacier which initially rounded it and then broke it. This fits with where you found it. So it is probably a metamorphic rock of some kind carried down from what is now Canada. The flakiness you describe I would guess is the result of the weathering that occurred while it was being transported by the glacier and after it was deposited in Michigan. I can tell from the photo that it has large mineral grains which is consistent with it being a metamorphic rock. The lighter colored spots may be a mica which has weathered. There appears to be rust stains in one area which may be from iron minerals weathering or it may just be from being in iron rich ground water/glacial water while weathering. Given the rough fracture pattern it is showing I would guess that there is a significant percentage of a dark feldspar.

So that is about as far as I can go in this one. A metamorphosed mafic (magnesium-iron rich) originally from Canada glacially transported to Michigan.

To say more I would have to see what the minerals in it are and what texture is showing internally.

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  • $\begingroup$ Thank you for telling us! You know a lot about rocks. We really appreciate your help. $\endgroup$ – Mineral Magician Aug 3 '17 at 19:53

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