My parents built a house into a hillside of glacial till in SE Wisconsin. The area is known for Silurian fossils. I find a lot of corals, crinoids, clam-like critters. Many of the fossils are in chert. Some are crystalized. I just pick them up as they erode out of the hillside.
I found this is a pile of sand/gravel/dirt that the contractor left next to the driveway. At first I thought it was a piece of plastic. I took it to the university geology department. A drop of liquid (vinegar? I can't remember) fizzed and she told me it was made of calcium carbonate.
It is an odd shape with straight ridges. I can't photograph under a microscope (although I'll give it a try) but the blue color is made up of very bright blue crystals that look like sugar - like the colored sugar you might put on cookies.
There are some layers of slightly shiny material on the brown side.