Any ideas what this could be?

[Upnorth]

Found this near a lake in polk county Wisconsin.  There seems to be a somewhat unusual pattern on it.  Can anybody identify this?

I can get more photos and dimensions if needed. 

Thanks a bunch!

[Upnorth]

Here's some additional photos with measurements.  Is this anything?

[Rockwood]

This brings to mind (not to be confused with an identification) a site I collect from. It's near the northern side of the Hardwood Mt. formation, now a poorly exposed nub, quite remote from the mountain. A few fossils there are fantastically well silicified, but most are disappointing when studied under magnification. Some are little more than what I take to be colloidally deposited silica, or the remnants of ash.